Boiled broccoli does not suddenly become dangerous or unhealthy. The main issue is that prolonged boiling can reduce some nutrients, while broccoli itself may cause digestive discomfort in sensitive individuals.
For most people, broccoli remains a nutritious vegetable—and the best approach is simply to prepare it in a way that works well for your body.
If you have a medical condition or take medication that requires dietary consistency, check with your healthcare professional before making major changes to your diet.