@movq If we're focusing on solving the "missing roots" problems. I would start to think about "client recommendations". The first recommendation would be:

1. Replying to a Twt that has no initial Subject must itself have a Subject of the form (hash; url).

This way it’s a hint to fetching clients that follow B, but not A (in the case of no mentions) that the Subject/Root might (very likely) is in the feed url.