Update No. 27 (November 2018) is now available on the MARC website (www.loc.gov/marc/). It is integrated into the documentation for each of the Online Full and Concise formats that are maintained on that site -- the Bibliographic format, Authority format, Holdings format, Classification format, and Community Information format. The documentation includes changes made to the MARC 21 formats resulting from proposals which were considered by the MARC 21 community since the publication of Update No. 26 (April 2018).
Full Text here.