Reddit archives (making them read only) threads after a year I believe. Why would Google re-visit archived threads? Did you see Google's crawler throttle crawl rate by response times?
Six months, and not back then. Back then you could comment on old threads. I wasn't there when they implement the thread lock, but I suspect it was related.
The reddit codebase is designed for recency. Interacting with old threads really trashed the databases, at least back then.
I'm not certain, but doesn't a 'read only' reddit page still get changed when a user who was in it deletes their account and posts? Often when a search engine sends me to reddit, the comments that probably contained the relevant information are long gone.
You might be right, in which case Google should offer a link to the corresponding Internet Archive wayback page (where the deleted content should still exist).
Failing that, there are browser extensions for both Chrome and Firefox that enable this functionality.
I used IA's browser plugin that does this for a while, but unfortunately had to stop because it had too many false positives (sending me to archive pages for sites that were still live.) An extension that does this properly definitely interests me.
The reddit codebase is designed for recency. Interacting with old threads really trashed the databases, at least back then.