If this battle is lost, then a dictatorship will begin.
The forum is already a dictatorship. Most internet discussion forums are.
Where is the right to self-determination?
You can always determine whether or not you want to keep using the forum, and what you want to do on it.
Can't we discuss mixers?
Yes, you can. Theymos has already stated that a couple of times.
Then came the end of freedom of speech.
Just as it is on Twitter/X - despite Elon's claims otherwise - 'free speech' here is just an illusion. There's really no such thing as free speech on the internet -- not so long as you are using somebody else's platform to do it. It really just means you have the right to say what you want to say in public and that the government won't stop you... even that has its limits.
How about starting a petition on the forum?
You are free to do that.