It’s a text based uml language.
Welcome to the forum, @Carina. I'm here before anyone else to inform you that one of the major rules here is to search before you post features since the devs are trying to crack down on duplicates.
This isn't exactly the same and is currently being possibly reworked back into Joplin after some bugs arose and it had to be temporarily disabled, but Joplin did support Mermaid, a language similar to plantUML.
Of course, if you want to possibly mimic it a tiny bit and are willing to mess with config files some, just yesterday, another user helped me with a similar issue (check the second link).
I’m sorry my mistake. I searched only through #features, so that it would be a task for code of summer.