It definitely started in the US, but with tiktok culture it has spread!
Ahmad is right that it is often used as a transitive verb (to slay something), like 'to slay a outfit/presentation/dance/etc.
Now the object is usually removed and we hear 'slay' used on its own, and it usually refers to outfits/hair and looking very good. If there is a tiktok about 'Who slayed at the Met Gala' I would know immediately that the video would talk about whose outfits were the best/who looked the best. It is also understood that it is a risky or very eccentric outfit that other people might not look good in, but the person who 'slays' pulls it off and looks great!