Element is just a client, while Matrix is the chat protocol. But yeah, I have recently switched my discord server to a matrix instance and its been pretty great. Some pushback by users who didn’t care about privacy or security, but overall the tech is solid. We didn’t like that discord was moderating private chats and they don’t offer any type of encryption.

IRC has also been working for me for at least 25 years

Mailing list! (/s… unless?)
And Lemmy/kbin obviously

Open source communities have been around longer than the internet, so they can manage fine without discord.

Also don’t you just join by submitting code or bug reports or other things?

Maybe a free and open source forum software. There are a few out there, maybe you used one before.

For synchronous communication? Element, Zulip, Rocketchat (or if you really must, IRC 🤮 ), just something opensource and privacy respecting.

CC BY-NC-SA 4.0

Anything else that can:

1. Segregate topics clearly, without stuffing all of it into a single stream
2. Can be queried from a web search engine.

Discourse is a great choice - it meets both criteria. Even phpbb meets the requirements.

Even Zulip is objectively better than Discord. It meets point 1 very well. I don’t know how well it does in point 2.

I think the point here is that anything that can be indexed by search engines or archival crawlers would be better, so not Matrix either. Forums, for example. Like what happens if the Discord community gets deleted due to whatever circumstances? All of that gathered knowledge will be just gone with no way to recover or search for it.