Only the Teimanim held of polygamy in recent history.
Moroccans and other non-Ashkenazim have living Jews who practiced it. It isn’t only Teimanim.
So yes, “exceedingly rare” in the sense that it was something practiced by a minority of one single Jewish ethnicity.
That is a mistaken notion. Au contraire. Enforcing monogomy is what’s practiced by a minority of one single Jewish ethnicity — namely, Ashkenazim. All non-Ashkenazic groups permit plural marriages. In fact, Chacham Ovadia Yosef zt’l was a very vocal supporter of it, and several times, on the record and available on recordings from his famous Motzei Shabbos shiurim insisted that Sephardim today should be permitted to have multiple wives. He was upset that the secular state outlawed it outside any binding halachic process. Indeed, the only reason it tapered off among non-Ashkenazim in general, is because the goyim/secular states most non-Ashkenazim move to in the last number of decades prevents them from engaging in it. From a purely Torah perspective it is fully permitted. If, as today seems very plausible if not likely, the various non-Jewish governments no longer pursue preventing plural marriages, then there’s every reason to believe non-Ashkenazim will immediately re-engage in their natural right they enjoy al pi halacha, already. And as has been pointed out, even for Ashkenazim Cherem R”G was initially setup with a built-in expiration date. Even though it was extended by later Rabbonim, current Ashkenazic Gedolim could decide it has outlived its usefulness and recognize that Rabbeinu Gershom himself set for the Cherem to expire and thus once again permit it.