Thank you Maxim for waking up the bug and giving an essential hint,
because I did not understand the bug anymore.

Indeed, without wrapping GDK_PIXBUF_MODULE_FILE as in my patch

guix shell --pure viewnior gdk-pixbuf webp-pixbuf-loader

does open webp files fine.

Wrapping it with the attached patch,

guix shell --pure viewnior

also opens webp files fine, but only after I have added
webp-pixbuf-loader to viewnior’s inputs.

But wrapping inhibits webp-pixbuf-loader when I don’t add it to inputs
even with

guix shell --pure viewnior gdk-pixbuf webp-pixbuf-loader

So wrapping apparently hard-codes which input formats there are.

Notably librsvg exceptionally does not need to be added explicitly.

Is it better to wrap viewnior and hard-code the supported image formats,
as I wanted, so installing only viewnior is enough?

Or better drop the attached patch and users need to install gdk-pixbuf
webp-pixbuf-loader as well?

I tend to prefer to wrap viewnior.

Regards,
Florian