Airmail has changed to a free app, with a subscription if you want push notifications or to use more than one email account. I don't really need those features but they also added a hideous badge that floats over the inbox, reminding you that Premium exists.
I do understand why they've done this instead of releasing a new, paid version of the app. There isn't really anything they could add to the app to make the upgrade worth it. I don't like subscriptions though. I might switch back to Apple Mail for a while, see how it is these days.
edit: Mail is still ugly.
edit 2: The MacOS version of Airmail hasn't updated yet, but presumably that's coming.
edit 3: They have an app called Airmail Zero which is the successor, but they're marketing it as a Gmail client, which I don't need. *shrug* I'm going to try Spark.