The asymptotics tag currently has two tags remapped to it: big-o and asymptotic-complexity. I can imagine the reason landau-notation is not amongst those is to distinguish between questions about asymptotics (in general) and the notation itself (and possibly its definition too); this seems to be what the tag description also hints at:
Questions about asymptotic notations such as Big-O, Omega, etc.
Big O Notation is an informal name of the "O(x)" notation used to describe asymptotic behaviour of functions. It is a special case of landau-notation.