A dependency being optional on a technical level doesn't mean that it's optional on a functional level for everyone. `libfido2` mentioned in the pull request is

A dependency being optional on a technical level doesn't mean that it's optional on a functional level for everyone. `libfido2` mentioned in the pull request is a great example: if your rootdisk is encrypted using a FIDO token, you need that library in the initrd; but otherwise you don't. By making the dependency optional and initrd generators aware of that optional dependency, people that need it will have things just work, while others don't need to install the library.