No, unfortunately, an employer (whether a private employer or governmental) is not precluded from enforcing a rule now or in the future simply because they have not enforced it in the past. Prior nonenforcement does not waive the right to enforce rules; and employers may, in any event, change rules at or about employment at will (except when restricted from doing so by the terms of a written employment contract), so that regardless of past rules or past practice, they could elect to now make it a key position.