I think Email Message-Id(s) only ever worked because typically you are exchanging emails with recipients you know and vice versa. It’s much easier to cope with the problems above, because you just ensure your client preserves the Message-Id. Email is a federated system, but by no means is it “decentralised”. You still have to send your email somewhere, not just post it on a website on your own server like Twtxt 😅

There are some subtitles differences like this that makes Message/Thread Id(s) not really that suitable IMO.