I'm convinced this is the most worthless tag on Danbooru. It's a prime example of the tagging bloat that's been getting worse and worse over the last few years.
This is a standard trope in anime art. It's practically as common as big eyes and tiny noses. It doesn't deserve to be tagged for the same reason we don't tag those things either.
This tag is present on a third of all posts from the last year. If it were tagged on older posts the way it is on new uploads, it would easily have 1 million posts. And if you search -eyebrows_visible_through_hair you'll see that even on posts where it's not currently tagged, in most cases it should be. If it were fully tagged I think it'd have at least 1.5 million posts, maybe 2 million. It'd be one of the biggest tags on the site.
Other tags that are this common - 1girl, solo, long hair, breasts - have clear use cases. This tag does not. It's not useful for filtering, and it's not something that's actually interesting to search for in it's own right, not in the way these other tags are.
Any tag that would apply to millions of posts needs a very strong justification in order to exist. Otherwise you're asking the rest of us to expend an enormous amount of collective tagging effort on something that provides no value in return.