The AFP protocol was deprecated in 2013. The AFP server was removed in Big Sur, so over five years ago. This is removal of AFP client support.

Apple's source is not public, but the protocol is still fully documented if someone wanted to create a new client and server. https://developer.apple.com/library/archive/documentation/Ne...

However, they'd be better off just creating a driver and server around the open source Netatalk implementation.