The Americans with Disabilities Act, the first comprehensive civil rights legislation addressing and granting basic accessibility needs of people with disabilities, turned 25 in 2015. The law includes sections prohibiting employment discrimination, public services, public accommodations, and telecommunications.
Note that the above instances do not mandate captioning. Governments and businesses must provide equal access to people who are deaf or hard-of-hearing, which may include ASL interpreters, CART providers, or C-Print note takers, among others. Captioning may be one of the requested actions.
In the case of online streaming providers, however, captioning is probably the only way to ensure their public business is accessible.