Feather-footed birds need plenty of DRY organic litter in their area so that they don't get into mud in the first place (sometimes easier said than done).
In *most* situations, they will preen and clean their own feet once the source of the muck has been covered with wood chips, straw, shavings, dry leaves, pine straw, what-have-you.
Some situations, like clay soil dried to adobe or snow/slush frozen into ice balls may call for human intervention. I haven't had to do that myself with any of my feather-footed girls (sandy soil being excellent for feathered feet), but my plan was always dry-cleaning in the form of busting up the hardened material so that the chickens could preen the feathers again.