> So, basically, don't use legacy clients on your account, cause they will degrade your overall experience.
Yes. As I said.
But let's not put a bouquet of roses on it: it's even worse than that, because you don't just have a degraded experience when using a legacy client. Using a "legacy client" -- of which there are many because of the XEP swamp -- will make all other clients behave wrong. And it is impossible[1] to tell which client (or server) to blame. Thus resulting in even the most modern and chose-the-right-XEPs clients regularly getting bug reports about lost messages... even if some other client is at fault.
If that's not a trainwreck I don't know what is.
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[1] inb4: nothing is "impossible". But "practically speaking impossible for an end user who has other things to get done today and a limited time budget for debugging a problem which is a $n$-dimensional matrix of their clients and servers in use", yes.
> but I can't tell you which of the three or for relevant ones are the most relevant
I can. Check out XEP-0359. MAM and some other XEPs depend on it.
Also, unique message IDs are not a feature. They are means to provide some feature. You will deal with different kinds of IDs when trying to ensure network reliability (0198) or when trying to implement read markers and delivery receipts (0333, 0184). Without telling us what feature would you like to implement (threading? attachments? something like 0367?) just spewing all different kinds of IDs from various XEPs starts to look like a FUD to confuse uninformed reader.
> multi device? Oh. My. God. It's bananas.
I can agree with opinion that the protocol is really "bananas", but it works. Basically 0280 solves this issue, especially when combined with 0198 and 0313 - the only situation where it breaks is if you connect with a legacy client that doesn't support carbons and set it a presence with higher priority than any other client. You'll still get the content of the messages on modern clients thanks to MAM though. So, basically, don't use legacy clients on your account, cause they will degrade your overall experience. Doesn't seem very surprising, especially given that you can't connect at all to most IM networks out there using outdated clients. My N900 with stock OS won't connect to Skype or Facebook at all, even though it tells me it can. It will to XMPP - with limited functionality, but it's better than nothing.
XMPP accumulated its great share of backwards compatibility weirdness, but it doesn't seem unmanageable and I believe that the way XEPs are handled made it better, not worse. I suspect that Matrix might be in worse shape when it reaches current age of XMPP. I would like to be wrong though, I wish them best.