(Note, I don't currently have commit access.)
The code in llvmorg-10-init-12188-g25ce33a6e4f is a breaking change for users of older linkers who don't pass a version parameter, which prevents a drop-in clang upgrade. Old tools can't know about what future tools will do, so as a general principle the burden should be new tools to be compatible by default. Also, for comparison, none of the other tests of Version within AddLinkArgs add any new behaviors unless the version is explicitly specified. Therefore, this patch changes the -platform_version behavior from opt-out to opt-in.