interesting. my back door has a deadbolt requiring a key on both sides. ..it's an old house.

Why have you left it like that? Yeah it's against fire codes, but for a very good reason: it's incredibly unsafe.

My house still has a deadbolt like that, too. They were popular for a while, especially on doors with large glass panes (so that would-be intruders couldn’t smash the glass and unlock the door w/o a key).