I'm not an animation historian and I've done no original research on the subject. If I had to make a guess I'd say some scripts must have been written because it's inconceivable that penny-pinching studio owners would have always, in all situations, resisted the common practice of live action, which was to use scripts. Also, just guessing again, I'd say that scripts couldn't have been very common. If they were, then where are they now? Why did books and articles written at the time (like "Art of Animation", above and below) emphasize story boards as the preferred way to write stories? Walt himself is on record in print and film saying that he didn't use scripts.
Bob Jaques says he owns a Fleischer script and Floyd Norman said he saw scripts being written while he worked at Disney's. Mike Barrier interviewed non-artist Bill Cottrell who wrote for Disney, and Mike put up Cottrell's script for "Cock Robin" on his site. Steve Worth was not impressed and says Cottrell's script was probably written after the storyboards were made, as a sort of handy synopsis or recording script. Mike disagrees and wants to throttle Steve, but Steve remains adamant. Here's an example of Cottrell's script, below:
It certainly looks like it was written after the story was already made and shot, at least as a Leica reel, but Mike says it contains things that weren't in the finished film, so it must have originated earlier. Maybe it was made from an early Leica reel. Gee, if a script this detailed and anal-retentive was written early, at the creative stage, it would certainly lead me to pity the poor animators whose creative input would have been zilch.
Steve says no scripts (meaning, I assume, creative scripts by non-artists that were more than just dictation) were written during animation's Golden Age. I winced when I heard that because there are exceptions to every rule, and I could imagine someone pulling out that exception from an attic somewhere. Mike says "Snow White" used scripts in addition to storyboards. Animation critic Charles Solomon says no scripts were used at Disney until "101 Dalmatians." I'm not an historian so I can't comment.
Myself, I think scripts are an absolutely terrible way to write animation, but I imagine that I can occasionally see the influence of non-artist writers in some classic films. "Lady and the Tramp" looked beautiful but the writing was full of cliches that are still used by non-artists today. "Aristocats" made after Walt's death, had an abundance of them. I simply can't imagine artists coming up with ideas as visually impaired as these. But maybe I'm wrong.