I still think sending a kind:5 followed by a new kind:1 with a reference to the previous version is the way to go. Clients that want can group replies to both posts together using some complex magic, but others can safely ignore anything that was deleted and just use the latest version and be happy about it.

Handling deletes properly is a complexity that we arguably shouldn't have introduced, but now that we have that we can easily build on top of it.