I don't really think there is a consistent answer to that, because your premise is questionable:

For example in the modern day we have millions of records about everything and getting a full picture is less of an issue.

There are still lots of modern subjects where sources can be scarce for one reason or another like war, negligence or just simply an archivist who has been busy with the shredder to make room on stuffed shelves. Or perhaps there never were any sources made for a particular historical event, like a private meeting between two people where no minutes were made or no later recollections by the participants written down.

Thus there really isn't a particular point in time where we can say that this is the point where from now on we have sources on everything, because even today you could still easily find subjects with little to no sources.