The main concept of openHAB is the eventbus. This is true for OH1 as well as OH2.
As soon as any command/state ends up on the eventbus everything is fine.
How about this proposal: We provide an OH 1.9 core runtime for all the left-behind OH1 addons that connects/syncs via MQTT (for example) with OH 3. I wanted to propose an OH-sync service anyway. I’m willing to write one for a new OH, the one for OH1 would need to write someone else though.
That way OH1 bindings work indefinitely (until all old hardware died out) and in OH2/3 nobody need to care about breaking changes for old OH1 bindings.