It's not a matter of not being used, as if it was some optional election. Florida law does not impose a duty to retreat. There's no special invocation to waive that duty, it just doesn't exist. It didn't apply to Zimmerman's case because retreat wasn't a factual issue in his case.
I doubt if any state or city had a written "duty to retreat" law. Leftist judges and prosecutors just pretended it was "settled law", and there were no grounds to appeal. It was a handy way to send innocent people to jail for daring to defend someone's life. Or another hammer to use in "negotiating" a plea bargain.
When states were forced to rewrite a lot of gun control laws because the Supreme Court said the Second Amendment meant what it said, legislators tried to close off the "duty to retreat", which was "unwritten law" with an explicit "stand your ground" statute, so that "duty to retreat" didn't sneak back in.
Personally, I'd like to see them write a model "duty to retreat" law that had to be voted on, and NOT something handed down like oral tradition. And nullify any "castle doctrine", replacing both with federal level (for consistency) minimal rules of engagement. They should make it simple enough, and sweeping enough, to engrave it on every new firearm. It would be a federal felony to shoot AT anyone without first reading this to would-be attacker in a clear and distinct voice in both English and Mexican.