You can find the Slot Restrictions variable in the class defaults of the follow blueprints.

BP_PlayerInventoryComponent

BP_StorageInventoryComponent

BP_ConvertInventoryComponent

Once you find the variable you can click the + to create a new entry and you will see 2 options shown in the image below.

The index is the slot that you are applying this restriction to so for example if you want the players first quick slot to only allow consumables you would set the Index to 0 as the slots start at 0 not 1. Then you would click the + button next to the categorys to add a new item category like below.

So the image above settings will restrict the first quick slot to only accept Consumables. If you want to allow crafting items in the first quick slot you can click the + again and add the Crafting category.

Above is another example this would restrict the last quick slot (the furthest one on the right) to only allow ammo items.

These restrictions can also be used on storage components and convert components in the same ways. If you need to know what a slots index is this image explains how slots are numbered in a inventory.