A recent find in an Austrian castle is aparently set to rewrite fashion history. Basically they have discovered two bras and a couple of pairs of skimpy string briefs which date from pre 1480 when the castle was extended. Prior to this the earliest date for a bra was the early 18th century, according to the article.
It does make sense that women wore skimpy briefs, particularly at the TOM when they had a need for padding. less washing and less potential for mess. As fashion history did not become popular/a serious object for study until the mid 20th century (the main driver was the movie industry), and mostly intimate garments were tossed rather than preserved, it makes sense that we do have a huge gap in our knowledge. People would not be comfortable talking about the subject, particularly in eras when even a glimpse of an ankle was shocking, so why would they keep those types of garments?
With bras, or over the shoulder suport, again it doesn't surprise me. Women are practical. I suspect over the centuries many different forms of suport were used. If you look at the history of undergarments in the late 19th century and early 20 th century (the sample size is large), the sort of undergarment required is dictated by the shape the woman wished to have. If you have a decollete with off the shoulder sleeves, you are going to need under the bust support, for example. The shape of the bust has changed as well. The pigeon breast of the 1890s and Edwardian age gave way to the flatness of the Flapper and eventually you get the pointed bras of the 1950s.
You can see the shape women have today is dictated to a certain extent by spanx and other support clothing.
The bra comes into its own with the advent of elastic. To make a correct fitting and comfortable bra without elastic is a highly technical job. Far easier to push them up sort of thing.
But it is one of those things, we can only have an educated guess about what Viking women or medieval women wore under thier clothes.
I also suspect that if I suddenly have my Viking heroines wearing bras and skimpy briefs all the time, readers will complain or be drawn out of the story. But it is something to be aware of certainly.