pgsql-hackers is the main mailing list for PostgreSQL development, including discussion of new features and submission of patches.
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pgsql-hackers, as well as modification of various settings, can be managed via https://lists.postgresql.org/; a PostgreSQL community account is required for this. A mailing list unsubscription link is included in the header of each mail (see here for details).
pgsql-hackers has been in existence since at least 1997, and probably since around the time Postgres95 came into being as an independent open source project.
The oldest message archived dates from June 1997 (Re: [HACKERS] Array bug is still there....), though from the context it is obviously not the first email to be posted to the list.