After a certain period of time they get locked, and at that time period they are usually buried so deep into the bowels of the forum that nobody cares about how cringey they were.
I've been a webmaster/forum-mod/forum-admin who has to concern himself with SEO and making my website (or my site owner's website) marketable. A key to this with forums is to never, ever, EVER delete anything unless it absolutely violates the basic rules of forum decency (Like posting Porn, for example). Even if the information is silly and obsolete and you think no one cares anymore, like "How do you make a Dos 6.22 boot disk in windows 95?", someone, somewhere might still want to know how for some pet project.
That way years down the line when someone googles a problem, your website might popup. Java-Gaming actually got a lot of traffic from me on google because of this. Back when I was learning how to program I ended up a Java-gaming lurker by accident, with no user account, just simply because I kept "falling" on this website through google.
Java-gaming is a goldmine of random programming knowledge, and if you delete it all I'm almost positive the website's traffic will crumble because of it. If you look at the forum stats, almost always there's about 10 times more guests than registered users, I bet they're almost all coming from google.