Should you leave the flagstick in?
Leaving the flagstick in may have some benefits but from a physics standpoint, there is zero evidence to suggest that the flagstick helps in any but the rarest of situations.
What the flagstick may do is occasionally reduce the length of a second putt and therefore possibly help reduce three-putts.
Of course, it also will clearly and substantially reduce the number of one-putts as science has proven.
It’s also clear from research that the fiberglass type of flagstick is the least detrimental (but still nowhere near as good as taking the flagstick out).