Matrix's approach also has problems that are well documented here: Why not matrix?
This touches on some things I've been wondering about in terms of how Fluxer will handle federation. Since one of the issues with Matrix is deleted content can persist via other servers that ignore such requests, which can include legally problematic content and spam (apart from things like accidental self-doxxing, etc).
Mastodon, which uses ActivityPub, is another example with this issue. Servers can use blocklists for unwanted instances but this seems to assume too much good faith as a default and ironically from what I've read introduces a catch-22 where deletion requests can't be propagated to blocked instances that store any pre-blocked mirrored data, so even if some of those instances would respect the request it causes the data to persist non-deliberately.
If Fluxer's approach was anything similar I'm not sure whether it'd necessitate some integrity check between self-hosted servers that they're running a known-good version of Fluxer that respects proper content sync. Obviously nothing would stop forks from just removing such a check and federating among themselves.
Maybe this is already a solved problem in some other federated platforms?
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Comment by @chocmake