Thanks for all the replies! It seems like broccoli can indeed cause some babes discomfort, but like sensitivities to anything else, not everyone has them. I'm glad to know it's not a myth, though.
It's curious; in mainstream sources I often see the warning about broccoli being potentially gassy for babes, but in crunchy sources, I often see warnings about babes being allergic to dairy, wheat, or other common allergens (this is something I hardly ever hear outside a crunchy environment). From a "breastfeeding education" standpoint, I do think it's important to let moms know that these sensitivities and allergies are a possibility
(so that if their babe does turn out to be sensitive to something they'll have an idea of what to do), but not
right to dwell on them in such a scary way as to suggest that breastfeeding is always necessarily full of dietary restrictions for mom. I definitely agree that a nursing mom should eat whatever she normally would unless
she detects a problem; and then to modify her diet accordingly.
Thank you, mamas, for educating me!