When Twitter quietly updated its developer policies to ban third-party clients from its platform, it abruptly closed an important chapter of Twitter’s history. Unlike most of its counterparts, which tightly control what developers are able to access, Twitter has a long history with independent app makers.

Today, Mastodon’s explosive growth in the face of Twitter’s collapse has made it a new UI playground, especially so on iOS. I’m following — and using — at least half a dozen excellent new iOS Mastodon clients, each of them distinctive.1 Mastodon has that small-nugget timeline nature as Twitter, but is a truly open platform. There are no limits to what developers can choose to do with the Mastodon APIs. There are, however, limits to what iOS developers can deliver to users: App Store review.