Inspired by this Q&A: How to handle falling down stairs?
Feather fall says:
Choose up to five falling creatures within range. A falling creature's rate of descent slows to 60 feet per round until the spell ends. If the creature lands before the spell ends, it takes no falling damage and can land on its feet, and the spell ends for that creature.
It has a casting time of 1 Reaction:
which you take when you or a creature within 60 feet of you falls.
In my answer to the above Q&A, I document three (stair) cases where the characters can fall down stairs and take damage.
From Tales from the Yawning Portal:
The eastern stairs are shrouded in dim light, which can’t be made brighter by any means. Any character who mounts the stairs falls down and rolls back onto the floor, taking 3 (1d6) bludgeoning damage.
From Rime of the Frostmaiden:
The stairs, which descend 90 feet to area G10, are slippery. The slippery ice is difficult terrain. When a creature moves on these stairs for the first time on a turn, it must succeed on a DC 10 Dexterity (Acrobatics) check or fall prone. If the check fails by 5 or more, the creature also tumbles 10 feet down the stairs and takes 3 (1d6) bludgeoning damage.
From Rise of Tiamat:
Steps are cut into the wall of the chute, creating steep, icy stairs that drop down 100 feet in a tight spiral. Because this entrance is seldom used, the steps become increasingly obscured by frost as the characters descend. At the 40-foot mark, a character must attempt a DC 12 Dexterity (Acrobatics) check to maintain a grip on the dangerously uneven footholds. Failure means the character loses his or her footing, sliding and tumbling 60 feet to the bottom and taking 21 (6d6) bludgeoning damage.
In the example from Yawning Portal the word "falls" is used to describe our descent of the stairs. In the latter two examples, the word "tumbles" and the phrase "sliding and tumbling" are used instead. As noted in my answer to the linked question, the module guidance for falling down stairs is very similar to the usual rules for falling.
Does feather fall work to prevent falling-down-stairs damage?