I don't know how widely used it is, but it definitely has its fanbase - probably mostly by people who've used it since ages ago.

From what I've read, "supported" is a difficult term for Slackware. It's development is mostly done privately and informal by Volkerding. There's no public issue tracker etc. Releases are done when Volkerding wants to/manages to do them.

It's not a distro for me and I won't recommend it as a daily driver, but Slackware is definitely interesting.

PS: I can't stop me from recommending NixOS/GUIX as another interesting advanced distro. Them being declarative, deterministic and immutable seems to me like the complete opposite to Slackware, which doesn't even do dependencie management.