I would like to clean out my database of old posts, but I would like to have them still available to google and other search engines.

Is there a way to simply export a bunch (10s of thousands) of threads as HTML?

Humm?
I guess you are thinking old school on ubb classic features.
Sirdude I believe has a mod for archiving. But it does not quite work that way.
As I recall it is just split out as somewhat as a sub forum. So it would still be in the MYSQL database.

well, the system knows how to convert a thread to HTML since I've used the "email thread" feature to email threads to subscribers.

Any ideas how we might tap into that system?

I will wait for some others to chime in.
But I think you are talking apples and oranges.
The email feature is just that, One at a time.
You would need to grab the email crop it and then save it with a unique file name to others. Then find a way to navigate to them

To me this would be a big mod.
So I guess we will wait for the big dogs to say yay or nay.

There are several feature requests for "Archive Forums" which is about what you're looking for.

The way an archive forum would work would be that posts would either be written out to html files or separated into another table in the database as to alleviate some of the load on the posts table. Users would be unable to respond to the threads, but all content would still be viewable on the forum...

okay, cool.
I would like to vote for vanilla HTML since that wouldn't require any special systems to make them available to the world at large.

Well, ideally the user wouldn't know that the thread is archived, it'd appear that it's just "locked" from being responded to