Technology

Matrix is probably the closest; it's federated, there are a dozen more-or-less actively developed clients, for just about every platform. You can self-host your own server. It has a lot of features.

It's not perfect; it has a lot of flaws, but there's slow progress. Things to be aware of:

• Despite it being "open", there's really only one server that supports everything, and that's Synapse. It's where all of the new features are tested and land first. All other (half-dozen) servers lag Synapse. And - IMHO - Synapse is an awful piece of software. It's a giant mess of Python, and it lumbers along like a bloated, arthritic hippopotamus.
• The way federation is done makes it very expensive to self-host. Everything's fine until one of your users joins - even briefly - a popular room, and suddenly your server's downloading 9GB of history and binary blobs. This can be managed, but you may as well quit your job and become a full-time admin, because
• moderation tools suck. Aside from the most basic banning, all mod tools are external servers you have to set up and configure and run in parallel. And the most essential tool - mjolnir, a "this account is a troll spam bot, so ban it site-wide" is still very beta-ish and it's nearly impossible to get any help with setting up or using it.
• It's really a rather heavy protocol. Lots of network traffic.
• bridging is better in theory than practice. Most bridging requires you to run your own server, and few major hosts provide anything more than IRC bridging, and even then you can't actually bridge to most of the biggest IRC networks because it's blocked by the IRC providers, because Matrix bridges are a major source of spam grief for the IRC rooms. And setting up a bridge between a Matrix and an (e.g.) Discord room is a fairly significant PITA, requiring a Discord mod to perform several steps.
• It does hand e2e encryption for DMs, but it's honestly pretty bad at it. It's a better Discord than a, say, Signal. Key management is a minor nightmare and it is both prone to breakage, and complex, with a lot of fairly obscure terminology needed to understand any but the most basic operations. Like, when it's working, it's fine, but as soon as anything goes wrong, you're in a world of pain. I came count the number of times I've lost entire chat histories with people.

And to throw up a challenge before anyone disagrees about that last point: try changing clients several times, across devices, and on the same client. Delete your client and reconnect (as if you lost your phone). See how long you can go before you hit a point where you can't get to your chat history.

It's a good alternative to Discord; it's categorically better than Discord. If you're not hosting the server, it's better than IRC; the user experience is simply undebatably better. It's a crappy IM platform. It needs far better mod tools, and some competitor to Synapse has to get out of Beta.

But if all you're looking for is an alternative to Discord and you ate fine with using a public service, it's a good choice.

Element (over the Matrix protocol). As someone who grew up on IRC, it is in no shape or form a replacement for Discord.

It makes the most sense to get off discord by being platform agnostic in my opinion, just going to wherever you can find clusters of the types of connections you want in whatever format works for you as long as the format meets your requirements like privacy or whatever else, if you can find the bulk of it in a single place that's great but not necessary.

IRC

Revolt Chat. Only problem is they limit you to 25mb unless you're self hosted.