This is definitely not enforced. I’ve downloaded multiple extensions in the past when I wanted to learn how they worked. All of them were obfuscated.

edit: saw the below comment and editing before this gets questioned. I’m not talking about minification. It was definitely obfuscation.

Does that only cover the background/web-worker or does it also include the UI parts (popup, content-ui, dev-tools...) ? That would make using something like React or Vue almost impossible.

There's a difference between minification and obfuscation, but again, I'm not sure how they adjudicate it or how much they enforce it.