This was solved. My system configuration was wrong. When I first installed Fedora, I had the SATA mode set to RAID, in the UEFI settings. It should have been set to NVMe. This worked with the 5.8 kernel, but did not with the 5.9 kernel, which is apparently more sensitive to this setting. Subsequent kernel updates and installations used the same configuration options, so testing with Rawhide or other later kernels didn’t work. When I booted into Rawhide live environmment from a USB thumb drive, the system booted as expected. The fix was to reinstall the system from the F33 Live installation media after setting the disk to NVMe in UEFI settings. Subsequent updates to the kernel via Gnome’s Software application have not introduced and new problems.
What tipped me off was that the system failed to boot as soon as the kernel was set to load, and that kernel parameters such as nomodeset did not make any difference. This suggested that there was a problem reading the disk, such as when accidentally deleting a partition near the boot loader can cause misalignment that prevents booting.
The Arch Linux Wiki for the Dell 9300 ([URL=“Dell XPS 13 (9300) - ArchWiki”]https://wiki.archlinux.org/index.php/Dell_XPS_13_(9300)[/URL]) instructs users to change this UEFI setting.