I highly doubt that Rav Solovetchik labeled himself a Modern Orthodox Rav.
Rabbonim do not and never gave heterim for women to NOT cover their hair, nor to dress in any way that was not tznius. It is to encourage the love of mitzvos and to CHOOSE to do more mitzvos with the proper kavonos and understanding of them and to understand why they are doing them. In other words to understand who they are and why they do what they do. A RAV can’t force a kehilah to do anything. They can only teach them and hope they follow the rules. Otherwise people will only follow the rules in public while breaking the rules in private.
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