Unfortunately RFC 1459 is just the shallow end. It both includes features that no-one uses anymore, and excludes widely-deployed features that everyone relies on. Lots of the de-facto IRC protocol isn't written down anywhere outside various client and server codebases.
The situation is not unlike that of terminal control sequences. The core VT10x functionality is more-or-less mostly in place, ish, in every terminal emulator, but some of the VT10x functionality is no longer implemented, and lots of subsequent useful functionality isn't well documented, so implementors are left to crib from other implementations.
Ircdocs.horse is an excellent resource but it is unfortunately not complete.
Irc also has a lot of -to me at least- obscure specifications for example involving channel modes. The functionality differs from server to server.
I have the impression that most irc servers are operated by programs with old codebases coded in C. New client implementations are continuing to get written, but I do not see a lot of server implementations around, probably because it is complicated to write.
The situation is not unlike that of terminal control sequences. The core VT10x functionality is more-or-less mostly in place, ish, in every terminal emulator, but some of the VT10x functionality is no longer implemented, and lots of subsequent useful functionality isn't well documented, so implementors are left to crib from other implementations.