From manual page:

              -r      Enables  the  restricted  shell.   This option cannot be
                      unset once set.

  But one can exit it with:

$ set +r

  One can work an a restricted shell with:

+ Create a ~/.profile that sets PATH to, for example, /rbin
+ Add symbolic links to /rbin, e.g.
  # ln -sf /bin/ls /rbin
+ Remove any unwanted environment variable or function from
  ~/.profile with unset.

  Above works with bash, on basic tests using .bash_profile:

export PATH=/rbin
unset command_not_found_handle

and user shell set to /bin/rbash

  Restricted shells overall are somewhat of a fragile solution,
but if the environment is really very small, it is a somewhat
safe option.