In my english elementary school there were definitely nine men's morris boards, but I've never seen anyone play it outside of elementary school.

It's not too uncommon. Nintendo included it in their collection of tabletop games for the switch.

It's well known in Germany under the name Mill, but I cannot recall the last time I've seen anyone actually play it.

My family (in the US) plays it a lot. My sister in particular "solved" it so far as if she goes first and doesn't mess up, will always win.

My dad made boards to give as gifts one Christmas.

Fun game.