wheelchair=* is used pretty widely, and wheelmap.org is an easy way to visualize the presence and absence of this tag. Is this the kind of thing you’re looking for?
People with disabilities have very diverse needs, so a single “disabled” key would probably be too broad. After all, adding wheelchair-accessible ramps to a train station helps wheelchair users, but if there is no tactile paving, then that same station probably isn’t very accessible to the blind.
To express these differences, we already use dedicated tagging standards for groups such as wheelchair users, people with vision or hearing impairments, and more. Having these specialized tags feels like a better solution to me.