I think neither "simplices" nor "simplex-theory" captures the topic that is currently described by "simplicial-stuff". If we want to rename it, maybe "simplicial-objects"? (even if taken literally is a bit more restrictive). I like a lot "simplicial-stuff" though, and I'd prefer to keep it.
That said, I think a tag-excerpt should definitely be added. What about the following variation on the math.stackexchange tag-excerpt?
For questions about simplicial sets, simplicial (co)algebras and simplicial objects in other categories; geometric realization, Dold-Kahn correspondence, simplicial resolutions etc.
(I removed model structures from the description because we already have a tag for those)
