Mask regulation etc will be different in every country, so it may become a huge mess if they are compulsory in BA, but not in Aeroflot, for example, or vice versa. Even now, when free movement between Estonia and Latvia started - masks are compulsory in buses in Latvia and not compulsory in Estonia, so you need to put on mask after crossing the border after 2h ride. These measures can't last long, otherwise no one will travel, if one risks to be fined or put in quarantine, because his temperature seemed high to random security person.