I have seen a list of schema identifiers in IfcOpenShell: https://github.com/IfcOpenShell/IfcOpenShell/blob/f01ae4a45b3fc294ab5bcd4de494cc446ac587dc/cmake/CMakeLists.txt#L462
set(SCHEMA_VERSIONS "2x3" "4" "4x1" "4x2" "4x3_rc1")
I don’t know if they are official buildingSMART ones, or specific to IfcOpenShell.
If buildingSMART does indeed have a list of official identifiers, then indeed, perhaps the vendors are just neglecting them or are unaware 
Would be interested to know the answer too!