In Metal (as well as OpenGL), there are 2 separate limits:

Maximum number of inputs (scalars or vectors) to a
fragment function, declared with the stage_in qualifier

This one is 32 on some platforms.
In GL, it's MAX_VARYING_VECTORS

Maximum number of input components to a fragment
function, declared with the stage_in qualifier

This one is 60 at least, which matches our limit.
In GL, it's MAX_VARYING_COMPONENTS

The implication of this is users not being able to have 40 scalars as an input to FS, even though we tell them 60 components are supported.