In #NAV2018 there are two new properties related to Table object, table fields, and table keys.
ObsoleteState property has three values: No, Pending, and Removed.
Note: Microsoft WhatsNewDynamicsNAV2018.PDF document has values as No, Pending, and Obsolete but in reality third option is Removed.
Removed indicates that the element is marked as obsolete in the database. However, the element is not removed from the database, but references to the the element in code are only allowed in upgrade codeunits.
Notice that The moment I set this property value as Removed, in the PROBLEMS window system starts showing list of objects which have dependency on this objects as problems (in above screenshot).
The ObsoleteReason property is an informational, free-text field for describing why the ObsoleteState has been set to Pending or Obsolete and preferably with guidance about fields or tables that have replaced it.