The Windows key is usually on the left side of the space bar but the key in the mock-up here is on the right, so I don't think they're replacing the Windows key.
I know kbin has this feature, it calls them collections. You can see some examples on the list of public collections here. They don't seem to federate though, so I'm guessing they're tied to the instance you make them on and can't be subscribed to from others.
I don't necessarily want more people, just the people I have interest in following. Most of those people didn't stick around here for more than a couple weeks after the Twitter takeover though.
That doesn't really apply to Lemmy's content though, since unlike Lemmy.world, Threads users won't be able to create /c/ communities. If a Threads user wants to post to a community in a way that Lemmy recognizes them, they'll have to post it to one under a Lemmy instance's control, or Lemmy users won't see a thing.
The Windows key is usually on the left side of the space bar but the key in the mock-up here is on the right, so I don't think they're replacing the Windows key.