In the previous allowed foods list flax was listed as a test food item. I believe able ate it throughout his treatment (I believe he needs to confirm this though). The main reason it is controversial is because it is a dried food item that contains molds. I haven’t tried it or eaten it on the diet; I say if it works for you then go for it. I would consider it to be a stage 2 item if you really want to be safe.
Keep in mind reactions may take days/weeks/months to happen. You might not feel anything the day you eat it, but if you eat regularly for months, eventually you might get a bad reaction.