The bottom line on that matter is: we don't know. It's never been tested in court.

What we do know for sure is that underlying base mechanics can't be copyrighted. You can't copyright the idea of "roll a die and add a number".

But what we as players of RPGs call "mechanics" generally include a large variety of far more creative works. Things that might be protectable by copyright because they're actually more like works of creative expression that actual "mechanics" as the law defines the term. Feats, class abilities, even the basic concept of "characters apply an ancestry at first level" might be copyrightable. If I had to guess I'd say that last one probably isn't, but the specific feats, class abilities, and ancestry statistics probably are protectable.

My opinion is that it's disappointing that Paizo is forcing everyone to open up all the content they've created if they want to use the ORC licence. I wanted to see a world where almost every creator would choose to use ORC to open up a significant chunk of their work, even if they hold back some of it selectively. But if they can't be selective, they may just choose not to use ORC at all, and end up keeping far more closed overall than they would be under a less restrictive ORC licence. This is particularly frustrating as ORC is a viral licence, so users who are offput by needing to make all their creations publicly available will also not be able to create any content for games that are ORC-licenced.