What are the Discord Alternatives?
There are many and it's hard to know what's good and whatnot with the discord alternatives. What is or isn't good and what is or isn't complete? There are many I'll talk about today along with the pros and cons of all of them.
The list
Revolt/Stoat
Honestly if you can look past its many issues it is a rather competent discord alternative, though with time it has been slowly getting worse rather than better. Overall it can be a decent choice if you're not looking to self host it, if you wish to self host it you'll need to rebuild all of the clients to point to your new instance.
Spacebar
Spacebar is still in development but is improving with time, and does not officially create any client anymore with Fermi at this point being that "official" client that one will use to use spacebar. It strives to be discord compatible, though this does not mean you can contact people on discord one spacebar, it just means existing discord bots/clients can often be modified trivially to use spacebar rather than discord assuming spacebar properly supports the API.
Polyphony
It really sounds like the dream, a FOSS and federated discord alternative with Discord API support. Though at this point there's nothing more to say than that. There is some work being done on the project, though nothing to really see as it's not complete enough for that, and it's trying to avoid the pitfalls of Matrix by thinking about the spec before actually implementing it, which this does have many advantages, it does mean that this project is a lot slower moving. I hope them the best of luck, though I don't know how long it'll be until this project comes to fruition.
Matrix
I have a lot to say about Matrix, though I'll keep it short. It is not a discord alternative. It lacks the ease of use of discord and has a lot more focus on channels rather than guilds. (or "spaces" as matrix wants to call them) And of being on there its been a poor experience of the classic matrix things you hear about on the internet, "Message can't be decrypted", the invites to very illegal rooms, the poor instance level moderation. I don't know what to say other than treat it more like a telegram alternative.
DCTS
Again, not a discord alternative. There is no way to create guilds as a user. It's very early on in development also so it's hard to say much more.
TeamSpeak
It's proprietary and has much more of a focus on voice channels than guilds and I'd personally say it's not a discord alternative either. I don't have much more to say about this.
Have I Forgotten Anything?
If I have please open an issue here, though keep in mind I have other stuff to do as well, so I might not get to it immediately.