Sorry if the link to my own site is frowned upon, but I haven't seen any other news coverage of this yet. The Tapbots team (probably most-known for its Tweetbot Twitter client) has started work on a Mastodon client, heavily based off its Tweetbot app. The TestFlight beta link was briefly posted across Mastodon, but it apparently wasn't meant to be public and the beta was subsequently closed. I managed to get access though and shared a few screenshots and thoughts.