> Now I wonder if mastodon client app developers will start filtering the allowed servers users can put in...
Any action of that kind would only fuel the alt-right persecution complex. Servers however do filtering all the time already, and that's how the Gab folks can be left out of view of the rest of the world.
Mastodon server instances already have blocked lists of "banned" instances. There's also a common ban list on github, so new server admins can join the federation in a "normal" way. The ban list has some pretty messed up instances it rightfully bans, but they either tend to be illegal or far-right. Far-left content is spread and encouraged.
Note: I'm not trying to take a side in this, and I don't try to identify with any particular "faction" in the federation space.
Any action of that kind would only fuel the alt-right persecution complex. Servers however do filtering all the time already, and that's how the Gab folks can be left out of view of the rest of the world.