> Does anybody know how many active projects are still left there?

https://sourceforge.net/directory/ shows hundreds of thousands, but I can’t see a good way to filter out inactive projects.

> And who wound up owning them after Slashdot?

https://sourceforge.net/about/leadership appears to indicate that it’s still associated with Slashdot Media. I remember when they were bought that the new owner spent quite a bit of time regaining the trust of the software community. No idea how healthy the company’s business or operations are now, but it’s impressive that they’ve survived this long.

I don’t really know what their USP is anymore. It might be fun to start a new project there just to see what it’s like now.

There's a fair number of active projects on SourceForge - a couple of names you might recognize are DOSBox, GParted, and qBittorrent - but what pretty much all of them have in common is that they're old. To a first approximation, no one's launching new projects on SourceForge; projects use it out of inertia, not because it's good.