Companies should never advertise unlimited time off. This is a myth. There's always a limit, and what this does is just impose extra stress when trying to guess what it is.
4 day workweeks mean about 52 extra days off, and yet not everyone wants a 3 day weekend. I'd enjoy some 3 day weekends, but too much will be stressful when dealing with the bottomless amount of things to do at a startup.
You can straight up say every year has 66 day PTO, but that means that people will feel guilty about not using it. But maybe this approach is the best.