Note regarding reopening votes: I've significantly improved the question based on the comments.
A passenger takes the flight: airport XXX in country A -> airport YYY in country B -> airport ZZZ in country C. The passenger is allowed by countries A, B, and C to exit/re-enter the airport YYY during their layover at airport YYY (which implies that country C is ok with the passenger leaving and re-entering the transit area in country B before flying+entering country C).
In that situation, can an airline prevent a passenger from boarding the plane during a layover because they left the transit area of airport YYY (e.g., to visit the nearby city)?
Note: This is not a theoretical question. A customer support employee from some airline was claiming that the passenger was not allowed to exit/re-enter the airport YYY during their layover at airport YYY, even though the passenger was satisfying all the regulations of countries A, B, and C to exit/re-enter the airport YYY. This made me wonder whether an airline can prevent a passenger from exiting/re-entering the airport during a layover and/or prevent the passenger from boarding the flight because they exited/re-entered airport YYY, or that simply the customer support employee was confused over the admittedly sometimes very confusing COVID-19-related traveling regulations.
Answer to comments:
- This question is so vague as to be unanswerable. Rules vary around the world, and COVID just serves to complicate things. Perhaps you could cite your actual source instead of 'A customer support representative of some airline'. – Gaspode the Indomitable 10 hours ago
The question is not vague and is not country-dependent: the question precisely asks whether an airline prevent a passenger from boarding the plane during a layover because they left and re-entered the transit area / the airport.