Reddit has grown massively in size in the last few years. Of these new people, the majority browse on mobile apps, where viewing the rules has more barriers, being hidden behind buttons.

This differs to desktop where you can see them on the front page of the subreddit.

So, people don't read the rules, and the standards expected for comments in AH, and then get their comments deleted.

A third possibility is that there are more submissions/comments being made, as the subscriber base grows. This would necessarily result in more deleted posts and comments, simply by volume, without indicating either a decline in quality or an increase in moderator zeal.

Rules do change at times, as do interpretation, but there hasn't been much shift recently on that. Not would I say there is a notable decline in discourse per se. The biggest difference is simply growth. More people inevitably means more rules breaking, the good ol' eternal September.

Add to this that the Admins have shifted the sorting algorithm some in the past year which has impact too. But it isn't a dichotomy. More removals doesn't mean declining quality. In fact both can increase alongside each other, and have the same root cause. Growth brings in new people unfamiliar with the rules, but it also brings in new people who can make the kinds of quality contributions we like to see, so it is two sides of the same coin.

Lately I see a lot of... Lazy questions. Like why did Byzantium fell if it was so big? Why not read something about Byzantium first draw conclusions and then ask something more specific from what you read?

Both I think. Rules were laxer here a couple of years ago (my first comments here would certainly have been deleted now), but there also seem to be a lot more one-line unsourced attempts at answers.

Rules have been strictly enforced for quite some time and it has only made this a better subreddit. The sunday digest is always full of great answers that may have taken a bit of time to write and therefore didn't show up in a timeframe suitable to many redditors. About once a week there is a meta post about removed comments. The assumption seems to be that good answers are being removed. They aren't. Reddit is a big community and lots of people happen across an askhistorians thread and think "I know the answer to this!", and maybe they do... sort of. I like to read the comments thread (reddit.com/r/askhistorians/comments). There is no end of rule breaking posts, single comments, link dropping, wikipedia references, and attempts at an answer which would be just fine in /r/AskHistory or r/history. There is a higher standard here, and the nearly million subscribers generally like the way the subreddit is moderated. I certainly do, and frankly I don't care that a small but vocal group of users don't like it. If this isn't your jam, you have the whole rest of reddit.

I've had a related question for a while that might offer another reason. It seems like there's a trend towards not showing deleted comments at all, instead of [comment removed]. I was wondering if that lead to even more people posting "why does it say there are comments?"

Untempered pedants Evil never sleeps. Be vigilant.