But do you need to know the past discussions? I don't really see why this would scare away newcomers to a group any more than usual.
These people annoy existing servers members and get angry replies because people are so tired of answering the same basic question multiple times a day.
Some servers have created processes where you can only access the FAQ channel until you prove your knowledge, and only then you can enter ‘general’. That clearly shows that something is wrong with content discovery on Discord.
From the numerous Discords I'm on, this problem is either fixed by making the members answering have a mindset to always help, or by using a bot with canned responses.
I've been in programming IRC channels since the late 90s. People asking the same questions as someone else asked the day before was the norm back then too.
In fact, Discord is better than IRC in this regard, because it has history in the client. You can easily search what's been written by a member or in a channel. I've used it many times to find relevant information in the Home Assistant server and similar.
You also get the history when you join, so you immediately get some sense for what kind of channel it is. Is it very active or not? Are they discussing relevant things or not?
IRC has none of that. Sure some built bots that recorded the conversations, but you'd have to know where to find the logs. When joining a channel it was blank. In fact I did this just yesterday when I had some SystemD question, and hopped on their Libra Chat IRC channel. Took me a few hours to realize this was a dead place.
That being said, I do think open source projects should consider alternatives like Gitter.
With the huge difference that IRC did not have chat history, unless you used a bouncer
Sure, you could set up bouncers and whatnot. But it's a barrier that Discord doesn't have.
Past discussions are invaluable for providing context. The worst thing that can happen to a (technical) discussion is new people repeating points that have been resolved already while the discussion has since moved on.
Admins and regulars alike may waste time and screen space repeating past conclusions, something a quick forum search avoids.
Your only hope is to have a curated, wiki-like resource that you point newcomers at, maybe with an IRC bot to easily reference it from the chat.