Old databases (from previous L-versions) keep legacy tables not used in L8.
There are no retention settings nor cleaning jobs for those.
They increase DB size and storage cost for no benefit.
I suggest a feature to truncate legacy tables from BackOffice or CLI.
Manual truncation is possible but very cumbersome and not likely to be performed spontaneously by partners.
A simple SQL script would also do the trick. That way there is not need to implement any UI or anything to Backoffice. A SQL script that looks back for the previous versions tables and just removes them entirely data and all. Could also need a log truncate as well but that is a bit more risky and complex.