With https://gerrit.wikimedia.org/r/#/c/163470/ there should be a new column on the externallinks table which is called el_from_namespace. Please add the new column to production to allow merging of that patch set in the future.
This is the first step for merge of that patch set. After the column is added the patch set can be merged.
After merge and deployment on all wikis the maintenance script "populateBacklinkNamespace.php --tables externallinks" needs to be run.
After the run on all wikis the config $wgUseExternallinksNamespaceDBField can be set to true (in mediawiki/core or in operations/mediawiki-config).