ubiq: I added a citation to my comment that you quoted a few minutes after posting it. You may have submitted your response prior to seeing my citation. The Darchei Moshe and Rema demonstrate Rabbeinu Gershom provided exceptions to the Cherem, such as if the wife couldn’t have children. This effectively establishes that prior to R”G’s Cherem no such exceptions were necessary for polygyny.
Also, the living examples for Teimanim, Moroccans, etc. who practice it are of regular Yidden.
As far as Tanach or Chazal, they didn’t always tell us everyone’s full marital statuses, etc. In other words, if Tanach or Chazal didn’t cite a second wife for Ploni, that isn’t necessarily an indication that Ploni didn’t have a second (or third) wife.
And in any event, the examples of Ashkenazim up to R”G’s time and the examples of non-Ashkenazim up to modern times, itself demonstrates that people without special status (other than, perhaps, being relatively wealthy, etc.) had more than one wife.