> Now I wonder if mastodon client app developers will start filtering the allowed servers users can put in...

That's precisely what Mastodon's creator suggested earlier today: https://mastodon.social/@Gargron/102184195834215862

Of course, he has no authority over mobile apps, so it remains up to app creators to decide. I believe there are three apps that share the majority of users (Fedilab, Tusky, Mast). Most popular instances will probably stop federating with them as soon as they can.

EDIT: Fedilab's creator already expressed that he'll do so to avoid the chance of being banned from the app store.