Should the government ration the amount of holidays people can take abroad in order to save the environment?

There is no need for international agreement. Each country can come up with its own mandate to control its citizen's traveling behavior as it is doing in form of destination recommendation/warning from the national travel agency. Also, the variant carbon intensity of holiday activities doesn't matter because in the end a cap on number of holidays people can take would reduce an equal percentage of carbon amoung emited for all economic classes.

It would require international cooperation/agreement to quantify and enforce the amount of carbon a person uses while on holiday. The carbon intensity of activities while on holiday will vary considerably. For example, a person spending their holidays camping will be using less carbon than a person staying in a luxury air-conditioned hotel. As such without international agreement it would be impossible to monitor what a person does while abroad, and so there would be no way of assigning a carbon value to their activities. With the need for internationally recognized standards to implement such a scheme would come arguments about the threat to tourism that such a scheme poses.