A protocol can mandate forced deletion. A particular client implementation may ignore it, or some users may circumvent it, so it would be a weaker kind of feature, but still a feature. And depending on circumstances it can be quite useful.
You are right, though I still prefer "weak feature" as a term :) There's enough value in such things. Cryptography crowd is concentrated on omnipotent Eve breaking ciphers, and that wrench from xkcd, but I dare to claim that majority of both commercial and private leaks happen just because well-intentioned users don't have enough capacity to keep track of all the things, and proverbially think twice. Features like "unsend", or timed deletion are indeed laughable on their purely technical merits, but do wonders saving users from grave mistakes anyway.
So if I was a dev on matrix/element and this feature came across my plate I would have to weigh it against features that I know can be implemented in a way which make technical and non technical people feel satisfied and better about the application.
In practice, in federated networks bad actors end up being blacklisted. It does not provide any "formal" guarantee, but… it tends to work fine enough. For this specific "deletion request" feature, of course it should always be seen as a convenience thing, and absolutely not about security.
As with many engineering things, it's tradeoffs all the way down. For instant messaging, a federated approach, using open protocols, offers what I value most: decentralisation, hackability, autonomy, open source. My options in this space are Matrix or XMPP. I have not attempted to self-host a matrix server, but have been very happy with my [prosody](https://prosody.im/) instance for almost a decade now.
Matrix has the appearance of being a drop in replacement for Slack or Discord, but the design decisions seem so compromised that the only explanation is they did manage to establish a (somewhat weak) network effect? It certainly is not a good look for an open source project to be running on Slack or Discord (free/cheap plans rugpulled or to be soon.) Then that leaves IRC, which has a network effect collapsing at a much slower pace.
I never got far enough to try hosting a matrix server, but reading the linked post -- Matrix definitely is not GDPR compliant. The combination of whatever end form of ChatControl the EU gets along with possibly hundreds of other laws across the world and individual US states makes me think the days of a public facing non-profit or small startup running a project like this are over. (Or maybe the future of open source is funding lawyers while the development is all done for pennies by AI?)
Not being chatcontrol compliant? That's a feature not a bug. Nobody wants that anyway. Just another stupid US lobby (Thorn).
A big organisation won't be able to run matrix for everyone no but that's the cool thing about it. People can run their own for smaller groups of people.
When I delete a message off Signal chat, the expectation is that the chat members are agreeing by social contract to abide by that.