On 2nd of September, Fujii Masao committed patch: Support ALTER SYSTEM RESET command. This patch allows us to execute ALTER SYSTEM RESET command to remove the configuration entry from postgresql.auto.conf. Vik Fearing, reviewed by Amit Kapila and me.

On 18th of December, Tatsuo Ishii committed patch: Add ALTER SYSTEM command to edit the server configuration file. Patch contributed by Amit Kapila. Reviewed by Hari Babu, Masao Fujii, Boszormenyi Zoltan, Andres Freund, Greg Smith and others. On the next day, Fujii Masao committed ...

Similar question has been asked many times on mailing lists and on IRC. Sometimes it's not mail sending, but file/directory creation, or something else that generally requires some interaction with "world outside of database". Can it be done? Sure. How, then?