This is demonstrably false. Previous administrations have not. It used to be normal to do things like keeping cabinet members appointed by their opponents or not put up a mocking picture of your predecessor in the white house.

> It used to be normal to do things like keeping cabinet members appointed by their opponents

This particular thing was not all that common between Presidents who succeed normally by election. I think the most recent was Robert Gates serving as SecDef across the Bush II/Obama transition, before that there were five kept across the Reagan/Bush I transition, and no more in the post-WWII period.

(It’s true that the pettiness level in this Administration is unprecedented, but this is not a valid example.)