I don't think all of them are doing it maliciously though. Maybe that's just the way their keyboard is configured if that's their native language and originally it's genuine users making a post and some other bot is just coming along and copy and pasting that one. Could be wrong but I'm not sure whether people are doing purposely to avoid detection.
No, here you are wrong. I'm using Cyrillic keyboard layout too but for to achieve what they do you need to switch between the two EN/BG (or RU) to change every single letter which has homograph or use Word like program to search and replace those characters.
I have seen this before in my University in order to avoid the plagiarism checker, some students has done that, but they got caught at the end.
If it happens in one word or two it can be taken as mistake but doing this for 15 and more words. Here is an example with proof >
Well I said I might be, but I've seen bots copy these posts with the foreign characters which are seemingly the original post, but where are the original posts they copy and pasted from?