You keep it and mark it "deleted", and you show "deleted" in place of the message. This is what most forums do already, independently of the underlying pagination mechanism.

So basically I'm pretty sure now that we need a new kind of SQL INDEX, as in, pagination index.

To do that aforedescribed accounting for us, and more.

It wont't be a new kind of index. It would be a new interpretation of OFFSET and LIMIT, which would ignore deleted rows.

Why would it? It will be rebuilt when a row is deleted or even marked deleted.

If we have an pagination index over (topicId, deleted) on postTime asc.