I talked to a Jetbrains representative at a conference about this. They said Fleet was/is an experiment in the realtime collaboration tech, which really bloomed during Covid.
They said it is no longer seen as a good direction internally, so not to expect much.
That's a shame to hear, I really would love something like Fleet from them where I dont have to install the umpteenth IDE flavor, or use one of their plugins with an IDE built around something else entirely.
Maybe things have changed since then, no idea.