These boards shouldn't be deleted, but they should absolutely be closed/locked/archived/whatever.
I don't think it makes sense to block them. First, because - fortunately - they are not suffering from spam. And it has a specific theme that can be used at any time.
Maybe moved to a less prominent location, but I still don't think it's something important at the moment.
What I think, and perhaps this changes the perspectives on boards and sub boards a little, is that there is more clarity about the subject to be addressed on each board. It's happened to me, and maybe it's happened a little to everyone, to have an idea for a topic and have difficulty choosing the right board to put it on. Therefore, I think the boards should be a little clearer in terms of what they exist for. Whether they are very or little active, young or old.
Either way, the cards that exist must continue to exist and not be blocked.
It's not at all contradictory to want inactive boards closed and want topics which are widely discussed across the forum such as security/privacy to have a dedicated board. That would simply be the forum keeping up with the times, rather than continuing to be stuck in the past.
That's why I say that we are not in a phase to eliminate boards, but to make more appear. This would help to better reorganize the forum, and make everything easier for everyone.
New times, new themes, so maybe it's time for new boards. However, the old themes are not over, and their boards must continue.
Either way, this is just my opinion.