And then there's the issue on switching between clients (e.g. PC and phone) or otherwise having client state lost - if the complete reading progress isn't saved server-side, then it becomes entirely impossible to restart reading; whereas with a receive-order log you can at least manually scroll to the message you read last and continue downwards.

That said, a similar problem already exists with edits, and a universal solution for arbitrary insertion should also be able to improve arbitrary edit notifications.