listen/notify was available in postgres 7.1 (and may well have predated it): http://www.postgresql.org/docs/7.1/static/sql-listen.html

7.1 was released in 2001.

edit: digging further, I found this:

    Bug fixes:
        [...]
        * the LISTEN/NOTIFY asynchronous notification mechanism now work

that's in the change list for Postgres95 Beta 0.03, released in July 1995: http://www.postgresql.org/docs/9.3/static/release-0-03.html