Discord

They explicitly state that their arbitration clause only applies in the US, probably because the US is one of the few major markets that lets them get away with arbitration nonsense and don't want to get slapped by the EU.

How's Mumble these days?

Matrix is quite similar to (older) Discord and therefore a good (easy to switch to) alternative.

Does Teamspeak has a chat that can be browsed later on? Else it's not useful for anything other than regular voice chat.
Rather I'd start renting a small vps and set up Matrix and if voice chat is not natively supported I'd also set up Teamspeak alongside.

Very scummy, but neither illegal nor bait and switch. For the vast majority of people Discord is free; there’s no lock-in or purchase, which is the necessary element for something to be bait and switch. You can walk away at any time without losing anything of value (as defined by the market and by law).

The new ToS goes into effect April 15th. You have 30 days after the effective date to opt out. Edited post for clarity

I'm pretty sure you just need to send them an email containing your discord username and registered email, as well as "I would like to opt-out of the arbitration clause in your terms of service." That's what I have done.

I have additionally emailed their help desk asking about what further details they may want from an opt-out request, we'll see if they respond. It is, of course, intentionally very vague.

It's impossible to apply this globally; many countries operate under different laws to begin with. This is also stated on the ToS's Limit of Liability section; quoting:

WE DON’T EXCLUDE OR LIMIT OUR LIABILITY TO YOU WHERE IT WOULD BE ILLEGAL TO DO SO.

Your best bet is to read the changes and see if any of it is applicable to your country. The linked section pretty much covers it up.

As a reference, in my country it is impossible for companies to force you to a specific arbitrator. Discord can't push this term on me as it would be illegal.