I don’t see anything out of the ordinary. Pacman will warn you about those optional dependencies. But since you did not install them explicitly, there shouldn’t be any problems. Packages listed next to grub are completely optional and do not break your bootloader whatsoever.
Edit: I tried to replicate what you had and it looks like you installed qemu (qemu-base is the default provider for qemu, the first one in the list when installing) and libguestfs explicitly. You should remove them with # pacman -Rs qemu-base libguestfs
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While SteamOS is open-source and everyone can build one for themselves, it is only officially supported on Steam Deck. They promised to release a generic version of it targeting more devices in the past, and this post hints that that day is closer.