You're conflating a few different things here.

The key icon was for a custom item pane section that BBT created when Zotero 7 came out but before it was possible to add a custom item field. It's still possible for plugins to create custom sections. BBT just doesn't anymore.

After we made it possible for plugins to add custom item fields, BBT added a custom Citation Key field at the top of the items list and removed the custom section.

We recently added new fields to Zotero, including an official Citation Key field. That made BBT's custom field redundant, and I guess @emilianoeheyns removed it.

We don't provide a built-in way for plugins to change the position of official item fields, but BBT could still do various things to move the official Citation Key field to the top of the pane, or it could provide an option to mirror the official field at the top of the pane.

But for your specific request, a custom section is still something plugins can create.

Or some other option mentioned, yes. I don't think the key icon is likely the way to go here.

I would love to hear about those -- I haven't found them yet. I understand an extra field would be easy enough to add, but I would prefer to just have the native field.

The section was indeed a stopgap because it wasn't feasible to put extra fields in the item box in earlier zotero 7. That is not coming back.

I have tried those but haven't had any success so far.