Mammoth is shutting down:

Sadly, we’re no longer able to reliably update Mammoth or operate moth.social at the level we want. Therefore we will be removing Mammoth from sale on the App Store. By the end of the January, we will also shut down Moth.social along with our other project, sub.club.

When it launched a couple years ago, I thought they were on to a good idea: pairing a new Mastodon client with its own server to make signing up easy. They had some funding from Mozilla, and they pivoted to try Sub.club for monetization, but subscriber revenue is tough when most servers are free.