Don't have the history graft to blame it now but the flag is only listed in profiles/desc and not in USE_EXPAND_VALUES_ELIBC.
Same goes for kernel_Interix.
commit c15b4a04b8aba66275507d591ff2d77ddf81b37f Author: Michał Górny <mgorny@gentoo.org> AuthorDate: Fri Jun 17 09:26:18 2016 Commit: Michał Górny <mgorny@gentoo.org> CommitDate: Fri Jun 17 09:26:32 2016 profiles: Add kernel_Interix to valid KERNEL values, #584292 commit 17a659ac3391e382fc7a2a9b9a0439d9b035bdb6 Author: Michał Górny <mgorny@gentoo.org> AuthorDate: Fri Jun 17 09:25:47 2016 Commit: Michał Górny <mgorny@gentoo.org> CommitDate: Fri Jun 17 09:26:32 2016 profiles: Add elibc_mingw to valid ELIBC values, #584292