For migration purposes this case is (hinted and) handled in: ..\MigrationTools\0811to1000Published.remap:340:\bAttrValue\b!DgnTagValue
The MDL C API has been internally converted to use the new DgnTag classes and as Jon suggests; mdlTag free functions are redundant and not necessary when migrating MDL C API code, but prefer to use the MDL C++ APIs whenever possible.
The defect Artur filed and referenced above will make sure to remove unnecessary existing library and header references, and make mention there is no need to free in the mdlTag create documentation.
HTH,
Bob