Thanks for your comprehensive answer
According to your explanation, if I understood correctly, the topic information will never be deleted from the database (except manually) and can always be recovered. But if posts are deleted, they will be deleted from the database and only their statistics will remain. The text of the posts is no longer available after complete deletion.
As long as a topic is not manually deleted from the database, we can have the statistics of a user’s deleted posts and topics and write the queries we need.
And in answer to question 6th, it can be said that deleting large topics has no effect on improving the speed and performance of the system. Unless they are removed from the database manually by accepting the risk.
Does the same thing happen with private messages?