What is the ruling on someone who forgot to shave their head during Umrah five years ago, then performed Umrah several times afterwards and shaved their head each time?

Shaving or shortening the hair is a ritual in Hajj and Umrah (definition), and one does not exit the state of ihram (definition) in Umrah except by performing one of them. If you shaved or shortened your hair in the second Umrah, you have exited the state of ihram from the first Umrah. The second Umrah becomes invalid because you entered its state of ihram while still in the state of ihram for the first. There is no penalty for any transgressions of ihram prohibitions committed during that period due to the excuse.