Here's a common verb that's not in dictionary: they say that the FAA grandfathered in Boeing 787 planes built before 2008, the year they started certification guidelines for lithium-ion batteries on new planes.
It is also often said that employees' benefits plans set up before certain dates are grandfathered in when the company changes policies.
It's funny how a usage can make no logical sense and sneak into the language not needing any permission or explanation.
Or was there an established history of this usage?
ok, there is history, rooted in 'older' southern voters
This does make logical sense to me. In a family, the grandfather may have rules that he lives by but time and different situations have made the current generation change his rules somewhat to fit their own lives. Grandfather, however, refuses to give up the rules he has used all his life. If this has not happened in your family it is very easy to imagine because that is the way grandfathers and grandmothers are. (I speak as a grandfather myself.)
It is then easy to see how this same dynamic can apply outside of the family. A group has lived by and with a rule for many years and to change it would be a hardship for that group but the larger population sees a need to change for the future so they allow the old group to continue with their rule either for a fixed period or infinitely. Everyone else must be governed by the new rule.
It compares directly with the family situation.