So I considered taking a stab at this question last evening, and while there are a few comments which do strike in the direction of an answer, I think that a META answer is also in order here.

First, and perhaps most important, there is no "list of WW2 casualties", at least in the sense of an official, agreed upon one. In fact, only a few countries can even provide more than generalized estimates of the number of their citizens killed in the conflict. Here is a table from John Ellis' "Statistical Survey" book on the War, and you'll notice everything is in nice, round numbers. So my point here is that if the recent bomb in Frankfurt went off and killed someone, Ellis isn't going to be going back to edit the next edition of his book from 2,050,000 civilian casualties to 2,050,001 civilian casualties in Germany. And of course, the estimates themselves vary, so you won't always see agreement on some statistics across various works, depending on what method a given historian uses, and/or what primary sources they give more weight to.

Secondly though, is the fact that there isn't going to be an Official Universal Definition of who gets to be a casualty and who doesn't. These statistics are originating from a wide variety of persons, groups, and governmental organizations, and while there will be a general consensus on the main points (i.e. someone killed in action is definitely a casualty), there is no world law preventing Dr. John Smith from writing a book in which he includes deaths from exploded ordnance decades later as part of the total casualty numbers. That doesn't mean that other works need to agree with him though. I have not, nor am I capable of, doing a survey of every work on World War II which touches on tabulating casualty numbers so while I suspect Dr. Smith is in the minority, I couldn't say quite how much he is, although it is likely safe to say he is in a small one, as no works come to mind which make a point of arguing for it.

So finally, the closest thing we're going to see to an answer is less about how historians will judge the deaths in terms of "how many died in World War II" and instead how the death is treated in practical terms. If someone is killed or injured by unexploded ordnance, how does their insurance policy classify the death or injury, for instance, assuming it includes any sort of provision that touches on armed conflict. In all honesty I can't say, but that is a question for a lawyer, or perhaps an underwriter, not an historian. Likewise, presuming the government has any sort of compensations which were tied to war deaths or injuries, it would be a matter of what their written policy is on that matter, which will vary country-to-country. There may be other practical considerations that I'm missing, but again, that isn't a question for an historian, as it has little bearing on casualties as an aspect of historical study. It is a question for someone who is versed in the German Sozialgesetzbuch or another appropriate law/program/group/etc in which practical effects are relevant, so again, a lawyer or a government official (and likewise for France, Belgium, Poland, etc. and so on). They might be able to give some semblance of an answer, but for an historian, there is no "official rules" on this, so it is a matter of opinion and argumentation.

I am not a historian and I've never posted here before. Please let me know if I've done anything wrong mods.

Let's start with who is considered a causality. That alone doesn't have an easy answer and is considered a controversial subject. Groups like the US Military, UNICEF, and WHO all have their own definitions. Then to make things more confusing further distinctions are made like 'civilian casualties' and 'casualties of war'. This is one of the reasons you'll see different estimates for the same wars. (p. 16)

I am not able to properly source this but it seems UNICEF counts after the war deaths as civilian casualties, like in the case of landmines. According to wiki (I know!) "3. Those dying, whether during or after a war, from indirect effects of war such as disease, malnutrition and lawlessness, and who would not have been expected to die at such rates from such causes in the absence of the war;" are included in their counts. I only include this because the numbers UNICEF releases don't make sense unless they are counting indirect after the war causes.

The nato definition for casualty is as follows:

In relation to personnel, any person who is lost to his organization by reason of having been declared dead, wounded, diseased, detained, captured or missing. Related terms: battle casualty; died of wounds received in action; killed in action; non-battle casualty; wounded in action.

The definition for non-battle casualty is (same source):

non-battle casualty / perte hors combat A person who is not a battle casualty, but who is lost to his organization by reason of disease or injury, including persons dying from disease or injury, or by reason of being missing where the absence does not appear to be voluntary or due to enemy action or to being interned. Related terms: battle casualty; casualty; died of wounds received in action; killed in action; wounded in action. 01 Sep 2003

There is an in-depth paper discussing the increase of civilian casualties of war from a professor at the University of Oxford and it seemingly includes civilians killed by landmines after the war as casualties.

The definitive answer seems to be that they can be, but most organizations do not include after battle casualties unless their injuries were sustained during the war. From the definitions I read it seems that social type groups include them (as evidenced with mines casualties which is a large problem even today) and government groups do not include them.

Edits: Added more sources.

It might be hard to give a singular answer to this question, since even now different sources have varying definitions of what they consider a casualty of war and what they attribute the death to.

One source in which such deaths might be counted towards the WW2 death toll is "Wars and Population" by Boris Urlanis. In it the author classifies bombing deaths as not only those inflicted directly by an explosion, but also deaths from homelessness caused by bombings even past the surrender of Germany through causes such as disease (but not hunger). However, in the book only deaths that have a statistically significant impact on the population are discussed as examples and since none such death is described and classified in it, a clear answer cannot be given in relation to this source.

As an aside the insurer "Techniker Krankenkasse" in Germany classifies the deaths of bomb squad members while disarming unexploded ordnance as work accidents. By their own definitions the death of unrelated personnel through such unexploded ordnance would also be classified as an accident. While this holds no historical significance, it is important in practice, which can sometimes influence sources being written today.

Edit: I'm leaving my post up, but by now there are better and more in-depth answers in this thread, so refer to those instead.

I would like to add to what /U/Georgy_K_Zhukov mentioned in his post to expand and look at modern unexploded ordinance, and used the following sources for my points:

Legacies of War

Washington Post long read

Aftermath: The Remnants of War: From Landmines to Chemical Warfare--The Devastating Effects of Modern Combat by Donovan Webster

After the first explosive ordinance was used during the American Civil War, there have been casualties amongst farmers, children, bomb disposal experts, and other unfortunate victims of these 'iron harvests'. It appears from the reading and research I have done that these victims are not counted as victims of the conflict the ordinance was utilised in, rather, they treated as modern victims of the ordinance outside of the realms of a specific conflict.

Two major examples of this, the Vietnam war and World War I (which I linked to in my sources), show that despite major efforts to clean up the battlefields, these are still millions of shells left in the ground waiting to be 'harvested', and that the legal liability for that ordinance has passed from the belligerent force who discharged or placed the shell/mine, into the hands of the government who controls the land. As /U/Georgy_K_Zhukov points out, there are issues such as indemnity and legal liability for the death/mutilation of the victims of the ordinance, and as such insurers are highly unlikely to insure an individual if the risk of an accident happening is high, which is where government/foreign aid steps in to ensure that the victims of explosions are compensated.

All of that brings me back to your point, which is that it appears to me through the research that while technically those deaths are directly related to the conflict in which the original belligerent utilised the weapon, unless there is a concerted effort by historians to reclassify the victims as part of the original conflict, those victims are treated as victims of an act of god, separate and unique from the original war.

To expand on the point about how historians measure war victims, there is nothing stopping a historian reviewing the death/injury figures in 200/300 years time and correlating the deaths from unexploded ordinance as part of the original war. /U/Georgy_K_Zhukov's point about the round figures stands, and while it is unlikely you will ever see an explicit figure, you could theoretically see a war memorial to the long tail victims of WW2, Vietnam, or Iraq who died decades and even centuries after the initial war being commemorated. This would boil down to a matter of scholarly insight, political will, and societal understanding, as it would probably happen after a shift in the perception of war and historigraphy. An example of this is how animals in war were commemorated with a stature in London in 2013; the nature of sacrifice and the victims of war shifted as British culture changed, and memorial stands as an example of a deeper understanding of war.

Ultimately, I would argue your question is possibly 50 or 100 years too early, and that when the last ordinance is removed from the effected cities we may see a cultural shift that sees the modern victims of the original conflicts as victims of the original war.