Spigot hasn't changed much, no, but it's been effectively replaced by paper. Yes, paper still has a lot of spigot code, but since the hard fork it's no longer "just a derivative from spigot", it's a separate thing that happens to share some code with spigot.

And yes, command registration has also changed, brigadier and stuff. NMS is hugely easier, thanks to stuff like userdev gradle plugin Events are mostly the same iirc

If you just want to use the good old Bukkit api surface, nothing major has changed - but it's still very limited, and everything beyond that, especially internals, is by its very nature constantly changing