Release notes for prior release branches can be found on the PostgreSQL web site. At the time of release of version 11, these were the supported prior release branches:
PostgreSQL 10:
https://www.postgresql.org/docs/10/release.html
PostgreSQL 9.6:
https://www.postgresql.org/docs/9.6/release.html
PostgreSQL 9.5:
https://www.postgresql.org/docs/9.5/release.html
PostgreSQL 9.4:
https://www.postgresql.org/docs/9.4/release.html
PostgreSQL 9.3:
https://www.postgresql.org/docs/9.3/release.html
Release notes for older release branches can be found at
https://www.postgresql.org/docs/manuals/archive/