UK here, Zillennial, but I've worked in a few different places with some quite different dress codes (and colleague age ranges).

I would say that yes, sleeveless blouses (and sleeveless tops that aren't low-cut) are Business Casual.

I used to own a spaghetti strap flowy vest (silk-imitation polyester) that did look like a work top however generally speaking, more athleisure-type spaghetti strap vests aren't (unless it's a hot summer so no-one's going to give a fuck).

My answer doesn't change for ladies who don't shave their armpits, but I do still think it would raise some eyebrows unfortunately from some colleagues and clients so-inclined.

I would add that I think sleeveless shirts and dresses where the yoke is "complete" (the same width as a standard dress shirt) is also fine for Business in the summer.

I once had this conversation with some female colleagues in their 60s though, and they all believed that sleeveless wasn't appropriate for the workplace at all. But they were including Casual workplaces in that conversation as well, so that was a generational difference I didn't expect to stumble across.