+++ This bug was initially created as a clone of Bug #1948586 +++

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.

1. 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.