I don’t think I’ve even seen a recumbent cyclist in real life without a high red or neon yellow flag on the back of the bike, presumably for exactly this reason.
The thing about those flags is they don’t address the problem of inattentive blindness. Drivers don’t expect to see something taht shape so don’t see them - flag or no flag. Secondly the flag doesn’t help with visibility where it really matters which is when the bike is really close to the vehicle on either side.
