Based on PostgreSQL 11 Beta 1 Released!:

User Experience Enhancements

Another feature that fell into this category was the inability to intuitively quit from the PostgreSQL command-line (psql). There has been numerous recorded complaints of users trying to quit with the quit and exit commands, only to learn that the command to do so was \q.

We have heard your frustrations and have now added the ability to quit the command-line using the keywords quit and exit and hope that quitting a PostgreSQL session is now as enjoyable as using PostgreSQL.