I think there's also something to the idea that it was a way to guilt people into work/vacation bleed-over. ("I'll just answer these emails on my day off because they are 'urgent' and because I can always take an extra vacation day later.")

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.