[ungzd]

Class diagrams, the most used of UML diagram types, are not exactly for system design, they're too low-level, and yes, "Round-trip engineering" that generates code from class diagrams (and class diagrams from code) was advertised as important feature of advanced UML tools.

[aflag]

Converting classes into UML diagrams can be usefull regardless if you classify UML as obsolete or not. It's a nice why to understand how a given class or interface is used throughout.