Dead Bird Pic by Tiia Monto, Creative Commons.
Tweetbot 6 has just been released and it has adopted the dreaded subscription pricing structure. It now cost 6 USD per year to use.
There are almost no new features in Tweetbot 6 worth paying for. It is therefore not a good idea to pay for the subscription just yet. Like all previous versions of Tweetbot, Tweetbot 5 is still available in the purchased apps section of the App Store, and is modernized enough that it will probably work for many more years (Tweetbot 3 still works, apparently).
To express your displeasure, download Tweetbot 6 to rate it one star.
If you have not bought Tweetbot 5 and are looking for alternatives that do not involve subscription pricing, Aviary and Fenix are two good Twitter apps.