Many fine longbow shooters wear arm guards. I believe Howard Hill wore one. The lower brace height and low string tension at brace height on some longbows makes it more likely that the string will rebound forward and hit the arm. I agree with Jim that this isn't necessarily a form problem.
If someone is hitting his arm with a recurve braced at 8", then it's more likely a form issue. Could be gripping the handle too far into the bow, rotating the elbow horizontally, hyperextending the elbow, or trying to open the fingers rather than relaxing them.