The inventory button you remember was the 'Persona' button. I miss it dearly. It disappeared with the older clients.
Hope you have a good time
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Instead of scrolling through the newer menus and editing buttons to make hotkeys, you can still use the old method. It will put the hotbutton on your cursor, and you can drop it where you'd like.
Example. Pet Attack would be /ho nameofbutton /command
/ho attack /pet attack
/ho backoff /pet back off
etc.