This is what agile is for. You have a backlog of work in priority order and it can change whenever you need it to. If you get an unplanned request, instead of saying "yes" or "no" go review the backlog and see where it fits. It's a true existential threat to the business, it can go straight to the top. If it's just a thing they want, it can go in with the things everyone else wants. If the ask is incomplete, you work on completing the ask before asking a developer to look at it.