I mean, it is a disability. At least in the US. The official definition of a disability by the ADA is : "An individual with a disability is defined by the ADA as a person who has a physical or mental impairment that substantially limits one or more major life activities, a person who has a history or record of such an impairment, or a person who is perceived by others as having such an impairment." You'd have to be kind of silly to say that speaking isn't a major life activity. And it definitely does "limit" your ability to speak, in that you are not able to speak how you want to. Thus, everyone who stutters is disabled, but with different severities. Someone with a minor limp is still disabled, it's just a very mild disability. Same concept here.