The manufacturers usually understate their RDs actual capacity by a significant margin. As a fairly inexperienced mechanic, I've never really considered "how small can you go" as question needing asking, only "how large."
FWIW, the Deore DX I have that's a couple generations after Deerhead works just fine on a 7-speed cassette I have that's 12-32. I bet it would go down to 11, but I don't have one to try, so I can't test that.