Some are, others aren't. The ones that are legal usually write it on the label but if in doubt, you can call your national federation and they will be happy to answer you.
A suggestion: Omega Alpha manufactures a liquid supplement called Chill meant to be giving 1 hour before exposure to stress that is totally legal and very helpful for "hot" horses. Cheaper than an everyday supplement and I personally saw a big improvement on a horse that got very nervous on the trailer.
Some of them are allowed. We have a policy of always calling the USEF hotline before we do anything. They can tell you if a product is ok to use. Perosnally I dislike calming supplements. I understand if you have a very unmanageable horse but if they are just a little hot I don't like them.
I have used Relax me and it worked far better than anything else I have tried! And it's eventing legal!
I am extremely sceptical of calmers but as a last resort I purchased Horse First Relax Me (Eventing Legal) and gave him 1 scoop each day for a week and then 2 scoops the day before a riding club show to trial it. My horse changed from looking like a wild stallion to coming off the wagon quiet, able to get straight on him to work him in, calmly ridden around a show ring with other excited exracers and stook bolt still in a line up. He won the class and got lovely comments from the judge on how well behaved he was for an exracer. Clearly I found this hysterical!
I dont feed him it every day as its not required, I just now feed a few days before a show and I certainly wont be stopping. At £20 a tub its not too costly either. In my opinion its worth a go. Hope this helps :)