The official X (formerly known as Twitter) account for the Attack on Titan anime teased a screenshot of Attack on School Castes, which is the post-credits scene for The Last Attack.
Attack on Titan: The Last Attack is a compilation film of parts 3 and 4 of Attack on Titan: The Final Season.
Attack on School Castes appeared in volumes 21 to 34, depicting a modern version of the Attack on Titan characters.
You can watch the Attack on Titan anime on streaming services like Crunchyroll.