For what it is worth, Discord does have permalinks to old messages. You definitely aren't going to find a 10 year old link today in large part because Discord hasn't been around 10 years yet, but I've copy and pasted Discord permalinks before to get points across and I have friends that use it for answering FAQs (have notes full of Discord links to copy/paste to answer common questions).

For what it is worth, there are users that use search on Discord (including the sorts that copy/paste the permalinks to answer FAQs). Maybe "nobody" thinks to "search first" is a truism, but even then I've seen plenty of forums full of the exact same users that never think to search first when they enter a forum and go directly to asking a question previously answered many times before. (Even when they arrive in a forum from a google search on that very question, people like/prefer the immediacy/power/relationship of folks responding directly to them than a canned answer.)

I'm not saying Discord is particularly great at some of these use cases or even the right answer, but they are very comparable and it's definitely nothing like burning books to chat over a phone. (That's a really broken analogy for several reasons.)

(ETA: If there is something to criticize Discord here for, it's much more the centralizing force problem that old forums used to be distributed across every style of web host in the old web, but Discord is a single for-profit point of failure for so many eggs to be put in one basket.)

forums don't necessarily need to block search engine crawlers to prevent bad behavior, that would most likely be done with restrictions on account registration or limitation on what new users can post (one forum I am on hides posts from new account until they've been reviewed by a mod)