I was having the exact same issue, but then with a Macbook 79 model keyboard (Macbook/Pro Intl) and the US International with AltGr dead keys.
One of the commenters in the FreeDesktop bug correctly notes that there might actually be little use for these symbols anymore, but since I'm not exactly an XKB guru I find it too hard to tell.
Anway, I added
to /usr/share/X11/xkb/symbols/macintosh_vndr/us and the error went away. Everything works as it should AFAICT.