Is there any regulation governing the accessibility of research papers? (to ensure they can accommodate the reading preferences of the users)
To the best of my knowledge, no such law has ever been passed (though I'm no legal expert).

Even if such a law was passed, however, enforcing it would likely be difficult or impossible:


How could one assess the accessibility of a paper that can only even be understood by a tiny community of specialists?
How could one ensure that an obscure or newly-invented notation can be translated to alternative formats?
How could one effectively force scientists to invest time and energy in compliance, when funding agencies can barely even get them to do quick and easy things like pre-registering medical studies?