@jas0n All instances of Mastodon run the same source code, unless it is one of the many forks, like Hometown. But none of the other fediverse server software out there uses Mastodon code. They are completely different projects with completely different codebases. What they have in common is that they speak ActivityPub.
There is not one implementation of email servers. There are specifications like POP3 and IMAP that many implementations adhere to. But Mastodon is referring to an implementation. There is only one, and it's right here.
There is (currently) only one living version of the Mastodon code base. It could be forked in the future, but it hasn't been.
There are other ActivityPub microblogging platforms (Friendica, Mbin, Pleroma, Threads if you count it) which users could also be running, and from the point of view of users it shouldn't be obvious what any other given user you interact with is using, but that's not got anything to do with Mastodon pull requests.
Thanks. I recall reading the blurb on the Mastodon site about Federation, decentralisation etc, positively encouraging people to fork and make Mastodon compatible servers. Obviously there are many front ends already so I’m amazed if no-one has customised their installation with a fork or made a competing impl in rust or something. Having the code centralised still seems like a flaw in the concept to me - Maybe the next stage of the fediverse is server code that can interoperate with the many different activitypub applications.