Down the line, this should pave way for PsqlForks to more cleanly convey why (most of) the given psql shortcut(s) don't work (and what else does).
As always, the psql/README always has the most updated status of any engine support.
$ psql -h localhost -E -p6543 -U postgres pgbouncer
psql (client-version:11devel, server-version:1.7.1/bouncer, engine:pgbouncer)
Type "help" for help.
pgbouncer=# show version;
NOTICE: pgbouncer version 1.7.1