This means that if you have other systems on your drive they will not be detected and added to the Grub boot menu since os-prober will not be run (the Warning text said it much better ).
You could add
GRUB_DISABLE_OS_PROBER=false
to
/etc/default/grub
and regenerate grub.cfg
sudo grub-mkconfig -o /boot/grub/grub.cfg
if you want/need os-prober
to run again.