“…my impression is a thread with the last activity dated back to (more or less) 24-month ago is presumably dead.”

Maybe that is where you start. For fun, I just searched “Hilton San Francisco”. I got a list of 1034 hits, but only 305 (ish) were from the last 2 years, meaning the last post was from 2006.

I would say that you could pick a previous date (2 years? 3 years?) beyond which the information is so stale as to be irrelevant, unless you are specifically searching for a past incident etc. The database of threads could be divided into current and archive, with standard searches done in the (much smaller) current database. Archive searches could be available as an option. Then, given a search for threads on a topic, one would end up finding and ‘bumping’ only relatively recent information.

This has likely been considered before. Maybe you need different archive periods for different fora, but it could be a way to both avoid very lengthy threads with little current information, as well as improve search performance.