I'm inclined to believe the claim that "unlimited vacation" was originally a way to get around having to compensate employees for unused vacation days when they leave the organization.
A big tech company changed the polices when I used to work there for the aforementioned reasons.
Which is a not so clever way of clearing vacation off the books also.
I will no longer work for companies that play similar games.
I urge others to consider doing the same.
I also got the impression that for some companies it was their founders' means to a loophole for remote work for themselves without instituting an actual remote work policy beneficial to lower staff, because who is going to tell a founder that they are taking too much vacation time, especially when they seem to be working the whole time? Whereas a lower staff member would easily be guilted out of taking anywhere near as much time to work remotely outside of the office. That's certainly something I think about every time I see a company mention unlimited vacations but not a robust remote work policy.