Didn't know about SHOW command. Are TABLES a reserved word?
There isn't going to be that much appetite for this just so that people can use PostgreSQL without reading our documentation: or the output of typing "help" at the psql prompt that says "type \q to quit".
I don't know how one would go about reasonably writing extensions for psql but that, to me, would be the preferred approach. Then people could install whichever compatability module they'd like.
I'll agree that exiting the program is a special case that merits consideration - and it has been given that in the form of the concession to the word "help" in order to get the reader unfamiliar with our backslash prefix a non-backslash way to obtain that knowledge apart from reading our docs. Under that premise I would accept (lacking compelling arguments for why its a bad idea) this proposal for quit/exit but am against anything beyond that.