Culture shock: "Smut-only extras" aren't a thing in English fandoms?

Just realized the "no backstory, just smut" extra trope (not quite "mob" level) is super rare in English fandoms! There’s not even a proper word for it.

It's so common in my language, I was genuinely surprised!

In my native language, we simply call them "passersby/extras" (路人), and everyone immediately knows what it means.