I did use checkrebuild and get the same warning. I’ve asked if it is due to my config or anything else but I didn’t get any answer. So, I’ve commented the line in my pacman.conf for now.
Maybe your request will show us the light…
Edit : even with the new rebuild-detector package (4.1.5-1) it’s still there
looks like upstream pacman/pacutils doesn’t support that option, but it is patched in on manjaro installs. manjaro has a custom pacutils that does not display that warning, but only (currently) on x86_64 installs.
created this issue. hopefully it will be addressed