[MattJ100]

> But why are these protocols frozen?

They're not "frozen". As mentioned in a sibling comment I wrote a blog post about this very topic - https://snikket.org/blog/products-vs-protocols/

Your point about optional extensions vs a protocol update isn't really as clear-cut as people think it is. To add a non-optional change to an open protocol in a decentralized network would necessitate blocking people from the network when you roll it out. That's not going to make for a good communication network.

The alternative is what XMPP does. The protocol evolves by adding new extensions, and deprecating old ones. Each extension generally has fallback considerations.

For example when group/offline media sharing was added many years ago, it was designed such that clients implementing the extension could render the media. Older clients, or clients that can't render media (e.g. terminal clients) simply display a URL.

The XMPP Standards Foundation annually publishes its "compliance suites", which (versioned by year) guides implementations on what they need to support. https://xmpp.org/about/compliance-suites.html