I'm also not sure. Like you, I wore some in the 80s. The collar made them look more dressy than tees, so I considered them OK for work. For many years now, I've seen them on men only. Interesting that they are making a comeback.
I own several regular tees. The plain solid ones are used as casual wear only, often with a scarf and worn with jeans, cardigans, loafers, etc.
I also have some nicer and more feminine tees, with embellishments or polkadots, and they are worn at work, with skirts, blazers/cardigans, ballet flats and a short necklace or perhaps a scarf (if not in conflict with the embellishment of the tee).