...it's been over a year and people still come up with new ones.

No, wearing a mask will not significantly reduce your body's ability to synthesize Vitamin D.

I can't find anything specifically debunking that, but here's an article from skincancer.org which states that masks do not provide significant UV protection and should not replace sunscreen, which I hope is good enough for most readers.

https://www.skincancer.org/blog/ask-the-expert-do-i-have-to-wear-sunscreen-under-my-mask/