Originally Posted by
clubeurope
As such, I understand that the border restrictions are very dynamic and things may change, but what position does this put the passenger into?
Into the same position a passenger is always in:
The one of sole responsible for meeting any documentation requirements for the trip booked.
The airline, any airline, can sell whatever is not illegal due to cabotage rules or below minimum connecting times. The rest is up to you.
(I admit that a warning message would be nice, but airline legacy booking systems would struggle with that...)
For instance, if I booked this itinerary, and Singapore (or whatever country) is still prohibiting transit flights between two airline companies, will one have a right to be re-accommodated to an alternative itinerary that is legal?
Not for the reason of it being "illegal" (to you). But many airlines currently allow free and/or flexible rebookings for new tickets bought.