I mean, people could just make an announcements channel that allows public messaging, that's not like a legal requirement of servers to make those restricted to posting, but they could totally make an announcement discussion channel, and even make a thread within the channel for each announcement post. Threads don't actually disappear, I don't believe, they go away from the immediate discord sidebar, but you can go to any channel and press the threads button at the top and see all previous threads intact.
And replying to someone's old message with context is totally possible. You can quote reply and @ them so they're notified and they can also tap your quote message to be taken back to the original message, helping them to remember context.
I don't really see discord as best at being a social media overall, but it is a decent community platform. I think the fact that reading older content is extremely difficult is true, and sort of a symptom of Discord's design, but that's the only real con I can think of.
And traditional forums don't theoretically disappear, but pretty much all of the self hosted ones I used to frequent back in the day are dead and gone, unless you count the way back machine. Actually, I'm curious, is there any software that allows you to back up at least the text contents of a discord server?