I, Donald Trump, affirm that if I do not win the 2016 Republican nomination for President of the United States I will endorse the 2016 Republican presidential nominee regardless of who it is.
I further pledge that I will not run as an independent or write in candidate nor will I accept the nomination for president of any other party
Rather more like a nonbinding pledge than sort of perennial binding deed. 2016 was years ago so it no longer applies. Already voided in 2016 by other candidates who never had any intention of "endorsing the 2016 Republican presidential nominee regardless of who it is."
How do you propose to enforce it; get the FBI to arrest him for lying on a nonbinding form?
He did not run on another ticket in 2016, so he has kept all of that.