Is this the slot that was talked about pre-inventory 2.0 to equip your custom items?
Or if not, does it serve the same purpose?
Because when creating a custom item I cannot put such slot as a item slot for the custom.

Utility slot holds tools and such, like macrobinoculars. It could probably be added, that system needs an upgrade.

Please submit a bug report that the Utility slot isn't selectable when spawning custom items, if one does not already exist.