When deleting a board, admins could be given a checkbox (off by default) to also delete all the individual topics attached to that board.
(or just the topics originating in that board? to account for cross-board workflows...)
Currently, at mediawiki, if just the board is deleted (not the topic), I can still see any topic that was on it, if I'm logged-out: e.g.
(which was created on https://www.mediawiki.org/wiki/User_talk:Quiddity_%28WMF%29/sandbox6 which I then deleted)
If a topic is deleted, I can only see the log entry if I'm logged-out, :
There are ambiguous expectations, about whether all topics should be automagically deleted, when just the board is deleted.