platforms is an argument the setuptools package inherits from distutils; see the Additional meta-data section in the distutils documentation:
Meta-Data:
platforms
Description: a list of platforms
Value: list of strings
So, yes, using a list is the correct syntax.
The field just provides metadata; what platforms does the package target. Use this to communicate to tools or people about where you expect the package to be used.
There is no further specification for the contents of this list, it is unstructured and free-form. If you want to use something more structured, use the available Trove classifier strings in the classifiers field, where tags under Operating System, Environment and others let you more strictly define a platform.
Wheels do not use this field other than to include it in the metadata, just like other fields like author or license.