After one or two false starts over the last... oh wow, probably a couple years... of getting automatic thread archival set up and running, it is now happening properly once and for all.
Threads older than 10 years are archived, meaning that they cannot be replied to anymore. Which should be fine, right? There are obvious exceptions for things like pinned threads, and case-by-case exceptions can be made individually if there's some weird reason to do so that I can't think of right now.
"10 years is a long time. Why not like 5 years? Or 2 years?"
Because setting it at 5 years would archive 98.3% of our 307k threads, and that felt like a scary high number. Nevermind 2 years, which would archive 99.5% of them. But the timeframe isn't set in stone so we can change it later if everything looks good.