It strikes me as well beyond the IT capabilities of most carriers and certainly not a priority given resource constraints during the pandemic, to constantly adjust every possible itinerary which might be sold to meet current regulatory conditions. Not only do various countries change their overall requirements on a regular basis, but issues such as transit and quarantine, change routinely.

As restrictions ease, the exact reverse will occur, e.g., itineraries which might have existed but which are not currently sold, may become "flyable."