While I'm definitely in the "single space" camp myself, and have previously been pretty annoyed by people who still use two spaces, I actually like how often I see the double spaces on Lemmy because it reminds me that there are quite a few people over 25 on here. I'm not quite 40, and I don't think I was ever taught anything other than single space, but I have friends that still do two spaces.
You can't do double spaces on Lemmy unless you're using some kind of app that doesn't automatically condense whitespace for some reason. In browser you can have any amount of spaces consecutively and they'll just get condensed into one.
Or people are deliberately using nonbreaking spaces specifically to make it not do that, for a gag. In which case, that's some real dedication just for a bit.
I'm learning apparently it isn't for Lemmy apps, which must have a habit of not using standard HTML renderers-- which seems like reinventing the wheel to me, but what do I know.
I would say every modern browser on Earth does this, but modern doesn't enter into it. Condensing whitespace in rendered web content has been consistent since the 90's.