It's becoming fairly normal at this point for more mature private companies, from what I can tell. If you've been at a company for 4-7 years with no IPO in sight, it can make some sense for the company to do a buyback. I know Uber has a tenure-based buyback program, Palantir did at least one in the last year or so, etc.
From the rumors I have heard, the Palantir buy-back is limited. My understanding is that most folks with options still haven't exercised the majority of them.
A company I worked for did the same, they raised some money and used some of it to purchase a portion of the employees' vested equity. In other cases the company's major investors were willing to tender to purchase employee equity to increase their stake.
