Bruno Philipe, writing on Mastodon.technology:
I’ve recently decided to make Mastonaut open-source. I no longer have the time and energy needed to maintain the app, and it has started to lag in features and functionality when compared to the Mastodon web client and other apps.
The app is now free at the App Store.
I don’t often check Mastodon from my Mac, but Mastonaut is by far the best Mastodon app on the platform. It’s a shame Bruno had to stop working on the project, but I’m glad it’s now open source so others can use it as a base for something better.