WHO stops short of advising proof of COVID-19 shots for travel

At this point asking for vaccine requirements is not a rational decision, so the WHO is completely justified in not issuing a recommendation. In one side because most people still don't have access to one, so adding an impossible requirement on top of rational measures (like complete isolation before and after the travel) brings no additional value; and in the other side because it has not yet been proved that being vaccinated prevents the person from being able to spread the disease, so replacing validated measures with the vaccine could lead to reintroducing the infection to the traveler's destination.

Test and isolate people properly at both points or restrict travel completely (depending on the situation), vaccines can enter the requirements only after enough data has been collected.

In addition to vaccines not being available to everyone, and the lack of proof that they prevent spread, we also have some medical agencies reconsidering their recommendations to vaccinate old frail people after a series of vaccine related deaths.

Although I don't think many old frail people will travel.