If you take in chickens who have lost feathers, one consequence is that a lack of butt feathers means their poop will often stick to the area around their vent, from which eggs, poop and pee all exit.
Those with crusty butts need regular cleaning. We just use a spray bottle of water and a towel. (Heavy duty "shop" paper towels work well, too.)
If the poop has been there a while, it'll be hard. In such cases, sometimes it's better to crush it gently in your fingers so it flakes off.
Be gentle with both techniques because if it's really crusted, it's easy to accidentally pull out feathers.
We probably go overboard by weighing the chickens every week to keep an eye out for any weight changes that might signal problems. So while we're weighing them, we check the butts of the few problem girls.
Once the butt feathers finally grow out, the crust problems generally cease.