I am not saying disability law originated in the DEI movement of today, I'm pointing at the through line between the passage of the ADA and the modern DEI movement. From the civil rights era in the 60s, to women's rights in the 70s, to the ADA in the 90s, to gay rights in the 00s, to DEI today. The principles behind disability accommodations -- access, fairness, inclusion -- are foundational to DEI as a broader movement. The ADA was an early expression of those principles in law; DEI later extended them into other social and organizational contexts.
And yeah, the ADA has received broad bipartisan support in passage because it's well understood even by partisans that disabilities affect everyone, so it's important to have protections in place.
What's not so understood by partisans is how those disabilities manifest, so since the passage of the ADA there has also been widespread pushback on what qualifies as a disability, and what accommodations are reasonable. THAT is a whole different conversation which, as someone who is disabled and covered under the ADA, I will say can be like pulling teeth to get protections guaranteed under the law. For example, businesses are often loathe to make physical accommodations like ramps and elevators, and there is often resistance to providing accommodations for mental health conditions or neurodiversity.
But DEI itself is about creating equitable access and participation for everyone. This includes people who are disabled, and at no point in time has DEI not included disabled people. Maybe for the terminally online right, who only focus on gender and race, but that's not what it's all about in the real world. Notably, DEI also has been a driving force for veteran employment (having dedicated veteran hiring pipelines is absolutely DEI). It's very common for people to do what you're doing now -- "All the accommodations I like and/or benefit me are sound law and not DEI; all the accommodations I don't like are DEI and must be outlawed"