How about people who, for one reason or another, can't get vaccinated for medical reasons?

Sure, in any reasonable system, they would be exempt from the requirement. But were they? If not, they almost certainly have legitimate grounds for a suit.

And it doesn't have to be a discrimination suit. It can be a suit alleging, for example, unreasonable requirements for employment. (Do you have to be vaccinated for measles to be employed by Washington? How about influenza? If not, then what's the rationale for requiring Covid and not the others?)