There's also now an open source recreation of the original client, all written in C# that actually uses a GPU so it renders 4k at 250fps instead of 800x600 at 12.5fps. It's a very mature and stable reproduction at this point.
(I've never used this but I heard it runs well)
Over 2k active players daily (edit: by active I mean there are over 2k players logged in and playing at any given time). The devs are incredibly active, and there is a lot more to do here than original UO.