I'm very OOTL when it comes to a lot of the major divides in online communities because thankfully I've managed to avoid most of the arguing. I've noticed a lot of instances say "no tankies" as one of their first rules and figured this might be the only place I could get a real answer.
Was there something specific they did to piss off everyone? If I had to take a shot in the dark, are they just people who are really pro-military?
The wikipedia is a pretty good write up. but generally you see it in European or multicultural spaces more than American centered, though it has entered the discussions, most no one knows what it means when they are using it.
People who use it that way are dipsticks, but I'm not surprised.
Dubcek was a communist, and so were most of those oppressed by the Soviet forces from which Tankies get their name.
There is a wild world of difference between the person who's like "Karl Marx was correct, but the USSR did a lot of extremely horrible things," and the kind of creep who openly stans Xi Jinping and Stalin.