The mod got updated and the config doesn't have the sections where you can change the slot they can be equipped in now, but that feature was kind of important to my modpack.

So, is there a way I can restore it?

You can use the 1.0 branch I made this change when I rewrote the mod due to the canisters occasionally getting nbt attached randomly and not being stackable anymore

This was the last version that allowed that
https://www.curseforge.com/minecraft/mc-mods/baubley-heart-canisters/files/2682612