@heluecht I don't think requiring some conversation URL would work with Webmention etc.

What (I think) I've done recently in !GNUsocial is to send (and accept) salmon slaps to everyone previously mentioned in the thread. That breaks as soon as someone who doesn't run a new enough version joins in though.

I think it depends on what kind of model of distribution is desired. I don't think relaying via the thread "owner" is desirable because if you have different policies on different sites you'll lose messages.

I personally prefer retrieving a notice by discovery when a notice comes in which refers to a parent which cannot be found in the local database.