It's sort of correct and sort of incorrect to say it's the only possible routing protocol between autonomous systems. Since many inter-AS connections are effect

It's sort of correct and sort of incorrect to say it's the only possible routing protocol between autonomous systems. Since many inter-AS connections are effectively made-to-order, they could use a different protocol, but in order to participate in the global BGP mesh, it would have to have semantics similar enough to BGP. Most notably, it would have to support the concept of AS-path. However I don't think there are any universal requirements beyond that. BGP isn't a single distributed algorithm like say OSPF - a completely separate instance of BGP runs between every pair of connected ASes and they share some data, indirectly forming a global system.