Arguably, you're legal rights aren't violated if you don't get admitted to a private party in the analogue world. Neither are your rights violated if someone disagrees with you or if you disagree with someone else.

In places where principles of government with respect for basic human rights are upheld by representative, mandated legal, executive and judicial bodies, you are free to contest that someone's opinion has damaged you before a court of law who will then assert your claim against a legal touchstone: the rule of law.

Libel, damaging the integrity, inciting violence against a particular, concrete group of identifiable persons or a single person, committing a prosecutable crime,... If a court of law finds that some party has overstepped their bounds, they will award civil compensation, or a corrective punishment (e.g. fines, prison time,...) None of that, however, is considered curtailing free speech rights unless you can make your case before a constitutional court.

The fallacy is assuming that Mastodon is (a) interchangeable with the fediverse (b) a single large unifying platform (c) a silver bullet solution which, contrary to centralized social media platforms, enables an utopian community devoid of human conflict.

Another fallacy is assuming that the public debate takes place in one big homogeneous public space. It does not. In the analogue world, there are countless clubs, associations, groups, factions, parties,... with their own meeting places, books, publications, journals, newspapers, periodicals and so on. For better or worse. The convergence of digital media into a few large platforms inadvertently have created such a perception, and capitalized on that perception as it served their private, business interests first and foremost.

Furthermore, subsequent scandals regarding large social media platforms created the backdrop for an alternative to come to the stage. Several technology outlets picked up on Mastodon. Created as a one-man effort by this young, idealistic developer, it fitted the narrative perfectly as an alternative that would break the dominance of big platforms over the public debate such as it is.

However, even though the software is published under an open source license, that doesn't negate intellectual ownership over one's creation. All it does is give license to use, modify and re-distribute the software under the same conditions. The original creator is still very much free to decide how to move forward implementing features, and espousing their own opinion freely. And you deciding to freely use of the software doesn't entitle you to a decisive vote on how the software evolves. For better or worst, the creator is perfectly free to say "Nope. Ain't doing that."

If anything, you are free to set up their own instance and decide to which instances to connect. You are free to fork Mastodon and change features to your own liking. You are free to build your own implementation of ActivityPub. You are free to promote your own implementation, set up your own network and recruit your own community. You are free to do all these things in many sovereign nations across the world who's constitution and legal framework provide those rights.

Of course, what makes all the difference is how people fundamentally treat each other. It's one thing to call out those who refer to others in derogatory, discriminatory and downright rejectable terms. It's quite another thing to assume that your free speech rights are being violated because another private person, or group of private persons, is unwilling to listen, let alone accept, your opinions.

Edit: I took the time and effort to clarify myself. I would like to ask for the courtesy to come up a proper argument in return, instead of a downvote which leaves everything and nothing to the imagination