Open protocols can evolve fast if they're willing to break other people
A while back I read an entry from Pete Zaitcev, where he said, among other things:
I guess what really drives me mad about this is how Eugen [the author of Mastodon] uses his mindshare advanage to drive protocol extensions. All of Fediverse implementations generaly communicate freely with one another, but as Pleroma and Mastodon develop, they gradually leave Gnusocial behind in features. In particular, Eugen found a loophole in the protocol, which allows to attach pictures without using up the space in the message for the URL. When Gnusocial displays a message with attachment, it only displays the text, not the picture. [...]
When I read this, my immediate reaction was that this sounded familiar. And indeed it is, just in another guise.
Over the years, there have been any number of relatively open protocols for federated things that were used by more or less commercial organizations, such as XMPP and Signal's protocol. Over and over again, the companies running major nodes have wound up deciding to de-federate (Signal, for example). When this has happened, one of the stated reasons for it has been that being federated has held back development (as covered in eg LWN's The perils of federated protocols, about Signal's decision to drop federation). At the time, I thought of this as being possible because what was involved was a company moving to a closed product, sometimes the company doing much of the work (as in Signal's case).
What Mastodon (and Pleroma) illustrate here is that this sort of thing can be done even in open protocols where some degree of federation is still being maintained. All it needs is for the people involved being willing to break protocol compatibility with other implementations that aren't willing to follow along and keep up (either because of lack of time or disagreements in the direction that the protocol is being dragged). Of course this is easier when the people making the changes are the dominant implementations, but anyone can do it if they're willing to live with the consequences, primarily a slow tacit de-federation where messages may still go back and forth but increasingly they're not useful for one or both sides.
Is this a good thing or not? I have no idea. On the one hand, Mastodon is moving the protocol in directions that are clearly useful to people; as Pete Zaitcev notes:
[...] But these days pictures are so prevalent, that it's pretty much impossible to live without receiving them. [...]
On the other hand things are clearly moving away from a universal federation of equals and an environment where the Fediverse and its protocols evolve through a broad process of consensus among many or all of the implementations. And there's the speed of evolution too; faster evolution privileges people who can spend more and more time on their implementation and people who can frequently update the version they're running (which may well require migration work and so on). A rapidly evolving Fediverse is one that requires ongoing attention from everyone involved, as opposed to a Fediverse where you can install an instance and then not worry about it for a while.
(This split is not unique to network protocols and federation. Consider the evolution of programming languages, for example; C++ moves at a much slower pace than things like Go and Swift because C++ cannot just be pushed along by one major party in the way those two can be by Google and Apple.)