Well that's a slightly pessimistic angle on it. Thinking about it my offer letter might've said something more like 'you decide', which is a bit more correct than 'unlimited' without the ominous threat of an undisclosed limit.

Perhaps I should have actually 'decided' and written something down if only for myself!

I find "unlimited PTO" enough of a warning signal that I think "not explicitly stated" is an excellent description of what it gives you. And, yes, at least one former workplace switched us from "25 days, plus Bank Holidays" to "unlimited PTO".

With pretty much everyone in my team complaining that we preferred an explicit limit than a nebulous non-explicit managerial capriciousness.