the most “drop-in” replacements I know of aimed towards users (not companies) are
https://matrix.org+https://element.io: Supports E2EE and voice + video chat, it works very well, but the onboarding experience of federated platforms is still confusing for some, and along with that they have to change knowledge of guild/channel type chats to space/room. I don’t use this.
https://github.com/msgbyte/tailchat: Very similar experience to Discord while not being it. It’s functionality is extended through plugins. Integrating all the services I use into one app is not as bad of an idea as it sounds, and I like the implementation very much, as do my friends. That being said, it’s, very bare-bones in it’s current state, and not much you can add through said plugins besides voice + video chat and bots. This is what I use right now.
https://github.com/spacebarchat: Very very alpha, but the end goal is very promising, that being complete backend compatibility with anything designed for Discord. The official client will not be finished anytime soon, so you have to bring your own Discord-compatible client and modify the endpoints to connect to and get any major use out of a Spacebar instance. Voice and Video chat still isn’t there yet, but most all other things work as expected. I keep my eye on it, and will probably use it later on in it’s more finished stages.