> Now I wonder if mastodon client app developers will start filtering the allowed servers users can put in...
This is not the duty of client applications. Banning instances is the duty of the operators of individual instances, and the instance(s) on which you have your account(s) are the instance(s) whose blocking policy affects you. This is a feature.
Most sane instances on the Fediverse already ban extremist and alt-right content, and Gab will be no different in that case - just another entry on the blacklist, next to https://freespeechextremist.com/ and https://rapefeminists.network and other instances who shamelessly abuse the right to free speech.
I wouldn't say they're abusing the right to free speech, they're taking full advantage of it.
If you don't like what they're saying, you have every right to not listen to it, promote it yourself, and you also have the right to start a movement to let people know this is going on and to bring awareness to the festering shitpiles those websites are.
Many people come to a conclusion that that the classical definition of free speech is easily abusable when enough people gather and utilize free speech to promote hatred, closed-mindedness and bigotry. Thanks to free speech instances we get hoaxes like the ones that way 5G is a mortal danger, vaccines are a mortal danger, women, Muslims, Jews and people of color are despicable, and many other interesting "100% verified truths and facts".
I know that the perfect solution for this problem does not conflict with free speech, but if that perfect solution (whatever it is) doesn't come soon enough, the world will get fed up with waiting and implement imperfect solutions, and limiting free speech is first on that list.
> classical definition of free speech is easily abusable
the classical definition of unfree speech is easily abusable, which is why free speech exists.
If an idea is bad, the best way to beat it is with better ideas.
Of course some bad ideas are good at spreading, or otherwise persuasive. But if you take the big picture view, the vast majority of really bad ideas and statements have been imposed centrally and have persisted by latching onto the levers of censorship and censoring all competing viewpoints.
The level of censorship and central control of information is already far too high in our world.
It might become their duty. Since Gab's apps are banned, and those apps would be used to access Gab's servers, I could see Apple/Google blocking Mastodon apps as well.
App creators might be forced to ban Gab in order to avoid getting banned from the app stores.
I wholeheartedly hope that this ban will not be limited to the AP protocol. gab is also available through HTTPS, and so popular HTTPS clients, such as Chrome, Firefox, Opera and Safari will also be forced to implement the blacklist.
It's trivial to use a Mastodon instance via a web browser. Are Firefox, Chrome, et al, going to be banned from app stores as well if they don't implement morality policing?
This is not the duty of client applications. Banning instances is the duty of the operators of individual instances, and the instance(s) on which you have your account(s) are the instance(s) whose blocking policy affects you. This is a feature.
Most sane instances on the Fediverse already ban extremist and alt-right content, and Gab will be no different in that case - just another entry on the blacklist, next to https://freespeechextremist.com/ and https://rapefeminists.network and other instances who shamelessly abuse the right to free speech.