Is your feature request related to a problem? Please describe.
Seems like an oversight, considering that shears, brushes, flint and steel can go into the tools slot.

Describe the solution you'd like
The Spyglass is allowed into the tools slot.

Describe alternatives you've considered
It can go into a regular slot inside the backpack.

Additional context
It's a tool, since you equip it and then right-click to use it.

You should be able to make a datapack that fixes it. I'll attach a template. Just add the items you want to acceptable_tools.json then put the folder in saves/[worldname]/datapacks. If you exit and re-enter the world, it should work. If not, turn on LAN and use the /datapack commands

put stuff in the backpack.zip
(just unzip it. I only zipped it up because GitHub doesn't like normal folders)

can't guarantee it will work though, because I've been having some trouble with a similar issue and only some of my datapacks worked :(

Fixed
https://www.curseforge.com/minecraft/mc-mods/travelers-backpack-fabric