Ok.... the "oxygen mask" excuse is total bull**** up to a certain point. If the beard is very long and bushy? Then yes it can POSSIBLY affect how much the mask seals to your face.
If the beard is neatly trimmed and not looking like a Taliban beard the mask will seal just fine.
How do I know? Bc I've had to wear the O2 mask when I was in the Middle East flying, had a beard, and needed to don the mask in a real smoke emergency.
I think many companies just have it as a policy Bc no one has really wanted to look into it, or stir that hornets nest.
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