Grub shows several entries in its boot-menu after upgrading from fedora 26 to 28 which no longer make sense:

  • Fedora 26 /dev/sda4
  • Fedora (0-rescue-lomg-hexadecimal-number), again referring to fedora 26
  • Advanced options for Fedora 26

There are no more kernels from F26 on the drives connected to this system, and selecting these gives a grub error that indicates this entry cannot be booted (forgot the exact verbiage). The rescue option starts the rescue image of fedora 28, not 26.
I have established that these stubs in /boot/efi/EFI/fedora/grub.cfg are generated by the /etc/grub.d/30_os-prober helper script. Disabling that (chmod 644) and regenerating the bootmenu gives the correct and expected result.

Somebody out there who can clarify whether commenting out this helper script is an issue and/or why these legacy entries were found by os-prober? I like to not just fix the optics but also understand what I have done... My setup is simple (single boot, neither SSD nor HDD contains other OS's)