As I have seen in my research, it seems like a lot of it has been if you want to feed a Bearded dragon Brussels, you would do it rarely. In addition to feeding a Bearded dragon Brussels rarely, here's a list on what you would rarely feed a Bearded dragon.
Broccoli, Brussel sprouts, Carrots, Cauliflower, Green peas raw, Rutabaga raw, Tomatoes, Pomegranate, Star fruit, Bananas, Rice brown long grain, Soybeans
For more information click the link above for more foods.
On more research, like stated above Brussels do have high ammounts of goitrogen.
Can Bearded Dragons eat Brussel Sprouts?
Yes, rarely, high in vitamins but also has goitrogen
Goitrogens – Inhibits intake of Iodine that then causes problems with the Thyroid Gland such as swelling.