SUB Plans have been around since the mid 1950's. They are primarily used by heavily unionized industries with the auto industry leading the pack. There seems to be a consensus that the Ford plan is the gold standard. They find their origin the Tax Code under Section 501 (c)(17). There are over 100 private letter/revenuerulings supporting various aspects of SUB plans, including the fact that benefits paid out under this plan are not deemed "wages" for the purposes of FICA, FUTA, SUI and Medicare employment taxes.
That would seem to confirm that there are at least two ways for a company/insurer to save money on severance. I've got to think there are more.
What blows my mind is why severance insurance (if, in fact, it does exist) isn't on corporate America's radar screen.