Here are two scenarios:
- A powerful enemy caster is concentrating on some high-level offensive spell (Incendiary Cloud, Earthquake, etc.)–one PC casts Banishment and the caster fails the save. Can the enemy caster maintain the spell from the demi-plane?
- A PC casts Expeditious Retreat and Blink on themselves: when transported to the Ethereal Plane, can they maintain the effect?
Currently, I’m leaning towards ‘no’ in the first case and ‘yes’ in the second, based on this last line from Blink:
You can only affect and be affected by other creatures on the Ethereal Plane.
In other words, as long as you’re in the same plane as the effect you want to maintain, then yes.