Lot of mailing lists offer the option to download mbox with old messages. So you can just download those for months/years when you were not subscribed.
(Also for example SourceForge only gives mbox files to administrators, not to regular users [0])
[0] https://sourceforge.net/p/forge/documentation/Mailing%20List...
Threads can also be downloaded individually.
And the search function is decent(ish) in the web interface.
Well, it is not perfect, much still, the hassle is one-time, so meh.