According to https://dev.gentoo.org/~ulm/pms/head/pms.html#x1-200003.1.4, underscores in USE flags should be considered reserved for USE_EXPAND. So I propose to rename the "luks1_default" USE flag in sys-fs/cryptsetup to "luks1-default". What do you think?
This USE flag will go away once cryptsetup-2.4.0 has been released.