[dragonwriter]

> What makes pybind a "flagship library"?

NumPy, SciPy, and friends are, individually and collectively flagship libraries, and depend on pibind11.

> If it was very widely used this bug probably would've been caught by people testing the alpha/beta/rc versions of 3.9.

They are very widely used, but “probably” means “do enough releases and you will find exceptions”. Welcome to an exception.