As far as I know, Near turned bsnes into a community project a while back when focus was put into other areas (Higan, Ares, etc.)
It looks like the nightly build autogenerated because the readme was updated to point to the community discord since bsnes.dev can't be updated.
Screwtapello commited the readme update to the github 4 days ago. They've been providing the primary updates to the project in recent months if the github commits are anything to go off of.
Jorpho, both emulators were originally created by Near(Byuu) with the focus of accurate low level SNES emulation. BSNES was the original emulator, but was eventually turned into a community project to allow for quality of life changes and interesting concepts (such as HD mode 7 graphics). Higan was the successor to that, which was becoming a multi console emulator aimed at low level accuracy. BSNES and Higan were both rolled into community open source projects. With BSNES allowing for more loosely controlled additions and Higans core focus being accuracy first and foremost (at least my interpretation of that). The descendant of both of those is Ares, which was similar to Higan but was a closed source project, with Near backporting improvements to Higan as time went along. A bit hard to follow, I realize haha.
As for Retroarch, well those are just the core engines that power BSNES, Higan, etc that are in a portable form that can be used for multiple programs. Retroarch and bizhawk utilize cores in this way to allow for individual development outside of their ecosystem that they can then bring in to their software. Or at least that's my interpretation of it all.
Hope this helps, friends.