The thong wiki page notes that a thong is "an undergarment that covers only a thin strip between the buttocks, exposing the cheeks themselves." I think the important distinction here is that while a thong can be a whole garment itself, it really describes a specific aspect of the garment (that is, the style of the seat). Clothing other than panties can feature thong backs: for example, leotards (post #367769), fundoshi (post #207893), and other clothing that defies obvious categorization (post #191478, post #393774). Also, males can wear thongs, and the "panties" tag probably isn't appropriate in such cases (I doubt that this tag combination will be a common occurrence on Danbooru, but hey, it could happen).

There are tags for thong_bikini and one-piece_thong and I think it would make sense for those tags to imply "thong." While those tags are useful, it would be cumbersome to use similarly specific tags for the above-mentioned examples; tags such as "thong_fundoshi," "thong_armor" etc. would be a bit silly.

Those non-underwear garments with that kind of cut are referred to as "t-back" in Japan, dunno about elsewhere.