what is "unused PTO" - I thought every company was now on the unlimited/zero PTO model
Unlimited PTO also discourages using PTO because nobody wants to be seen as the person taking the most vacation. And there are therefore no useful guidelines about how much is reasonable or allowed. A written or de facto company policy of "if you take more than 2 weeks of PTO per year, you'll be seen as abusing the system" is not unlimited PTO, it's an excuse to not pay people.
That depends on the management. I took more vacation at Netflix than anywhere else (where we had unlimited PTO). But the management made a point of talking about their extended vacations and making sure all the VPs took at least a few weeks of vacation every year to set a good example.
There was no stigma to taking vacation.
But in both cases, the CEOs actively encouraged PTO. At my current job, people take PTO regularly (several people at my department have taken roughly 3-4 months of PTO over the course of the past 12 months, and were promoted). What matters isn't time-in-seat, but whether tasks get done.
They actually have difficulty hiring people though because it wasn't a great place to work under the previous CEO a few years ago, so the Glass Door score is pretty low, and it's been slowing edging up over time. But it's gotten to the point where about half of the people who leave for greener pastures end up coming back within 6 months.
I'll take one for the team then. When do I start?
This fallacy needs to die. When I was at GE everyone in my blast radius took at least 1 month per year. Many took much more than that. There was no stigma.
It may not be true everywhere, but every company I've worked in that had "unlimited" PTO had far fewer vacation days taken than companies with a limited allocation of days.