After a little digging it looks like this was caused by LP#1698206: Eliminate Staged Search.

unapi.biblio_record_entry_feed replaced unapi.bre in WWW/EGCatLoader/Util.pm.  Util.pm is continuing to send 'pref_lib' but it's in the wrong parameter (title).  Also, unapi.biblio_record_entry_feed and unapi.metabib_virtual_record_feed don't know what to do with pref_lib once they get it.

I was able to restore pref_lib functionality with the following hack on my test server.  This has been very lightly.  

*******************************
OpenILS/WWW/EGCatLoader/Util.pm
*******************************

--- Util.pm.orig        2018-03-20 15:45:29.232569315 -0400
+++ Util.pm     2018-03-20 15:47:02.348572718 -0400
@@ -406,7 +406,7 @@
             $unapi_args->{site}, 
             $unapi_args->{depth}, 
             $slimit,
-            undef, undef, $unapi_args->{pref_lib}
+            undef, undef, undef, undef, undef, undef, undef, undef, $unapi_args->{pref_lib}
         ]}
     );


******************************
unapi.biblio_record_entry_feed
******************************


--- unapi.biblio_record_entry_feed.txt  2018-03-16 16:44:39.928533700 -0400
+++ unapi.biblio_record_entry_feed_pls.txt      2018-03-20 16:02:46.125997337 -0400
@@ -1,6 +1,6 @@
--- Function: unapi.biblio_record_entry_feed(bigint[], text, text[], text, integer, hstore, hstore, boolean, text, text, text, text, text, xml)
+-- Function: unapi.biblio_record_entry_feed(bigint[], text, text[], text, integer, hstore, hstore, boolean, text, text, text, text, text, xml, integer)
 
--- DROP FUNCTION unapi.biblio_record_entry_feed(bigint[], text, text[], text, integer, hstore, hstore, boolean, text, text, text, text, text, xml);
+-- DROP FUNCTION unapi.biblio_record_entry_feed(bigint[], text, text[], text, integer, hstore, hstore, boolean, text, text, text, text, text, xml, integer);
 
 CREATE OR REPLACE FUNCTION unapi.biblio_record_entry_feed(
     id_list bigint[],
@@ -16,7 +16,8 @@
     creator text DEFAULT NULL::text,
     update_ts text DEFAULT NULL::text,
     unapi_url text DEFAULT NULL::text,
-    header_xml xml DEFAULT NULL::xml)
+    header_xml xml DEFAULT NULL::xml,
+    pref_lib integer DEFAULT NULL::integer)
   RETURNS xml AS
 $BODY$
 DECLARE
@@ -44,7 +45,7 @@
     xmlns_uri := COALESCE(transform.namespace_uri,xmlns_uri);
 
     -- Gather the bib xml
-    SELECT XMLAGG( unapi.bre(i, format, '', includes, org, depth, slimit, soffset, include_xmlns)) INTO tmp_xml FROM UNNEST( id_list ) i;
+    SELECT XMLAGG( unapi.bre(i, format, '', includes, org, depth, slimit, soffset, include_xmlns, pref_lib)) INTO tmp_xml FROM UNNEST( id_list ) i;
 
     IF layout.title_element IS NOT NULL THEN
         EXECUTE 'SELECT XMLCONCAT( XMLELEMENT( name '|| layout.title_element ||', XMLATTRIBUTES( $1 AS xmlns), $3), $2)' INTO tmp_xml USING xmlns_uri, tmp_xml::XML, title;
@@ -78,5 +79,5 @@
 $BODY$
   LANGUAGE plpgsql STABLE
   COST 100;
-ALTER FUNCTION unapi.biblio_record_entry_feed(bigint[], text, text[], text, integer, hstore, hstore, boolean, text, text, text, text, text, xml)
+ALTER FUNCTION unapi.biblio_record_entry_feed(bigint[], text, text[], text, integer, hstore, hstore, boolean, text, text, text, text, text, xml, integer)
   OWNER TO evergreen;

*********************************
unapi.metabib_virtual_record_feed
*********************************

--- unapi.metabib_virtual_record_feed.txt       2018-03-20 16:08:57.992353316 -0400
+++ unapi.metabib_virtual_record_feed_pls.txt   2018-03-20 17:08:31.399917068 -0400
@@ -1,6 +1,6 @@
--- Function: unapi.metabib_virtual_record_feed(bigint[], text, text[], text, integer, hstore, hstore, boolean, text, text, text, text, text, xml)
+-- Function: unapi.metabib_virtual_record_feed(bigint[], text, text[], text, integer, hstore, hstore, boolean, text, text, text, text, text, xml, integer)
 
--- DROP FUNCTION unapi.metabib_virtual_record_feed(bigint[], text, text[], text, integer, hstore, hstore, boolean, text, text, text, text, text, xml);
+-- DROP FUNCTION unapi.metabib_virtual_record_feed(bigint[], text, text[], text, integer, hstore, hstore, boolean, text, text, text, text, text, xml, integer);
 
 CREATE OR REPLACE FUNCTION unapi.metabib_virtual_record_feed(
     id_list bigint[],
@@ -16,7 +16,8 @@
     creator text DEFAULT NULL::text,
     update_ts text DEFAULT NULL::text,
     unapi_url text DEFAULT NULL::text,
-    header_xml xml DEFAULT NULL::xml)
+    header_xml xml DEFAULT NULL::xml,
+    pref_lib integer DEFAULT NULL::integer)
   RETURNS xml AS
 $BODY$
 DECLARE
@@ -44,7 +45,7 @@
     xmlns_uri := COALESCE(transform.namespace_uri,xmlns_uri);
 
     -- Gather the bib xml
-    SELECT XMLAGG( unapi.mmr(i, format, '', includes, org, depth, slimit, soffset, include_xmlns)) INTO tmp_xml FROM UNNEST( id_list ) i;
+    SELECT XMLAGG( unapi.mmr(i, format, '', includes, org, depth, slimit, soffset, include_xmlns, pref_lib)) INTO tmp_xml FROM UNNEST( id_list ) i;
 
     IF layout.title_element IS NOT NULL THEN
         EXECUTE 'SELECT XMLCONCAT( XMLELEMENT( name '|| layout.title_element ||', XMLATTRIBUTES( $1 AS xmlns), $3), $2)' INTO tmp_xml USING xmlns_uri, tmp_xml::XML, title;
@@ -78,5 +79,5 @@
 $BODY$
   LANGUAGE plpgsql STABLE
   COST 100;
-ALTER FUNCTION unapi.metabib_virtual_record_feed(bigint[], text, text[], text, integer, hstore, hstore, boolean, text, text, text, text, text, xml)
+ALTER FUNCTION unapi.metabib_virtual_record_feed(bigint[], text, text[], text, integer, hstore, hstore, boolean, text, text, text, text, text, xml, integer)
   OWNER TO evergreen;