because of the search function - will it exist
In that case, you could use built-in Google bar. In true, HTML would be done really only for indexing purposes.
But if we could only put the old forums in some "archive" state and this way to freeze them, kind of separating them from the active part, it will be the best.
That's exactly I am talking about and probably it will be the big final code for miniBB in general.
Of course, there is now, too, a workaround for that: to have more than one copy of the same software with different databases and separate links.
At last, it is good working solution!
If the old archives are in HTML, will it be possible to maintain the search function as in the active part?
As I told already, HTML is just for "offline" versions or search indexers. As we plan this kind of archiver, it will create separate tables and put old data in them; later, you can use this tables as usual forum (for example, in the script itself variables $Tf, $Tt, $Tp will be replaced).