For reference, I’ll answer the subject question. implicit none (type,external) is a Fortran 2018 feature. implicit none(type) is the same as implicit none, disabling all implicit typing. implicit none(external) requires that any procedures referenced either have an explicit interface or be declared EXTERNAL. You can combine the options.
Compilers supporting this may have a command line option to imply implicit none (external) in the same manner as options to disable implicit typing.