I found a List in Shared Components that had a Build Option.
This list was used as the top menu in the User Interface settings.
In 18.x (1+2), I'm unable to clear the Build Option selection due the following error:
Navigation menu lists cannot be deleted while they are referenced in the user interface attributes.
I'm not trying to delete, I cleared build option, then hit apply changes.
Temporarily removing the reference from UI allowed me to save the change.
This was not a problem in 5.1.3, though it made me wonder what use this attribute is at the List level. Individual list elements, maybe, but what else do we get here beyond categorisation? We can't toggle the list used within a region.
Scott