@Catherine2511, I agree that it is utterly frustrating to see key features of the custom promotions disappear without letting hosts know about that upfront.
I am facing the same issue like you, one has to wait at least one day until the dates are no longer grayed out so a new promotion with a new discount can be added.
Like many hosts, I loved the concept of being able to adjust the %, in particular when it comes very close to an unbooked date or period. And in the past, one had the ability to make those adjustments promptly, without delay and as often as deemed relevant.
And it is not only the slider that went missing. @Bhumika fails to understand that whenever a day or period within a current promotion gets booked, it is no longer possible to just update the promotion with a different discount for the entire original period, but only separately (if at all) for the period prior to the booked dates within that period and for the period after those booked dates. How cumbersome!