Is your feature request related to a problem? Please describe.
No

Describe the solution you'd like
I would like to be able to get some of the different options for skins without the skin being visible. I would love the sponge or diamond/netherite upgrade, but I dont like the way they look.

Additional context
Perhaps make a separate slot for skins and "skin upgrades effects", or a toggle that allows you to say "always show leather skin" inside each backpack individually?

Currently the only option is to hide backpack entirely - you can do that by entering backpack settings (cog icon) and clicking eye icon (when backpack equipped).

The latest update for 1.20.1/1.21.1 implements new model system for backpacks, utilizing standard json files. Therefore you are able to use mod Transmog

https://www.curseforge.com/minecraft/mc-mods/transmog

This mod allows you to change appearance of items, along with backpack. It works for equipped backpack, so you can take any backpack with ability and change the visuals to any other backpack of your choice