You could have this flexibility with accrued PTO. I've previously given employees additional, off-the-books time off because they needed and deserved it and when we all got laid off they still got the remaining PTO paid out.
Edit: add section on flexible alternative
Companies have a lot of flexibility in deciding how the accruals work. But, as you said, the important part is that when it accrues, at that point it's a legally-obligated right for you to either receive that time or be paid for it.
In this sort of environment, I've never seen a company claim "unlimited".
how does that work?
I generally try to work at places that value my contributions but do not try to micromanage me. If I want/need to take time off, I do. If at some point I'm taking off so much time that they no longer feel that I'm contributing enough to the company, then fine they can let me go.