Not broken, but strange since it should be working on latest Safari.
https://caniuse.com/mdn-html_elements_optgroup_disabled
I think it may be a Safari bug.
Came here to mention the same thing. Very well be on me, tho. I’m using the Brave browser (is it safari-powered?) and on iOS 18.7.9, which is the newest my old iPhone X supports.
Your link states it's not supported in iOS Safari at all, even though it has been supported in macOS Safari since 2013.