Mail list archives. But at the point where the oss community I was part of switched, I had had a backlog of 8 years of mails.

You check the mailing list archives - those usually have a web based interface.

Admittedly I was being facetious: OP said mailing lists are better than a forum, but for searching the archive you need a forum-like interface anyway.

Its a bit easier as it can be read-only & static generated. Though there are some attempts to make it possible to reply using the Web interface, like HyperKitty /Mailman 3.

Google? Actually back in the day, gmane was the shiznit.