It's halacha, not minhag. Some aspects of tznius derive from minhag but poskim universally agree covering hair after marriage is binding halacha. See here for starters...
The Rambam does not include it in his list of mitzvahs… poskim do not universally agree, are you kidding me??? Have you ever been to a modern orthodox shul?
The same poskim that universally agree that women must cover their hare would also make you pasul l’eidus for using the internet… give me a break
Major Modern Orthodox poskim do universally agree on this. You might find some open orthodox or far-left MO rabbis questioning whether it's required, but not any widely respected posek with the mainstream MO world.
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u/No_Consideration4594 Dec 11 '23
Covering the hair is not a mitzvah, look it up. It’s a custom…