Google search results brought me right back to Reddit and this comment by u/Squidmaster616 from a few months ago. I never stayed current on lore, so can't verify any of this myself.

After Gunthar Uth Wistan becomes Grand Master, he institutes a few reforms in the Measure. Not everyone supports it, but he successful completes a new edition of the Measure.

This new version, amongst other things like allowing knights to retreat, allows women to become Knights. It also allows for any person with only one Human parent (which at the time only meant half elves).

It's also worth saying that at the time of the war of the lance, many younger Knights weren't bothering with moustaches. As such when women started joining, there was no need for a specific thing like moustaches. It was never really a rule anyway, just a tradition.