What ways are there to disrupt spellcasting in 5e?

[ProsecutorGodot]

There are examples of it happening in the PHB already (Light Cleric's Warding Flare, a GOOLock's Entropic Ward, or the Protection Style). Such a reaction goes after the attack has started and before it is resolved.

Those are exceptions, and they don't interrupt the trigger (in the sense of preventing it from happening) they change the outcome.

Please, could you tell us when the "X foe starts casting a spell" trigger finishes?

This is reminding me of when Crawford said "There is no taking the attack action, the attack just happens"

Seems relevant in the sense that there isn't an intent for such a trigger to exist.