This can be a nasty carrying cost for companies.
I worked at a place that went from the standard accrual of vacation to "a minimum of 20 days/year, no carry-over."
To keep employees from engaging in open rebellion, they had to offer two tiers: either 1) you agreed to the new plan and gave your accrued time back, or 2) you keep your accrued time to date, received no more, and could join the new plan only when your vacation time balance hit zero.
The new employees, including me, quickly picked option one. The long-timers all picked option two, sat on their time, and cashed out when they went to other jobs.
Earned vacation time is earned - companies trying to reclaim it are acting dishonestly and need to be avoided at all costs.
As for how it gets used, apparently it's very bimodal and "everyone" is either a "keeper" or "flirts with having none".
Personally I don't have a problem with "unlimited PTO" but (big but) there really does need to be a culture of people taking a reasonable amount which I would define as a month or so.
Which is why unlimited was born, because now you never "earn" any so you never "lose" any, and thus can never accrue any either
See the government saw employers with "use it or lose it" policy as "screwing over" the employee, so they ban it believing all companies would just accept they would have to allow accrual of PTO / Vacation time
Reality is that is not tenable for most companies and many people (like me) would just rack up years of time because we never take any off (when Possible I just cash out any PTO / Vacation I get, but many company stopped doing that as well)
So in response to the government banning employee lost their access to guaranteed number of days, instead how have this "unlimited" which is also an impossibility.
IMO "unlimited" is many many times worse than use it or lose it with a guaranteed number of days every year
It's also why many companies hate carrying vacation days over - they don't want someone to save up three months of vacation that they earned at a low wage from getting promoted, and then receiving it in cash when they quit at a higher wage.
Given the choice, an employer will always prefer to keep payroll costs predictable.