Bicycle helmets exist since about 25 years only and are therefore NOT historically correct (with the exception of non homologated historic racing helmets as on the picture). Modern helmets FIRMLY DISTURB vintage appearance.
However, there is no doubt that riding protected by a modern and homologated helmet increases safety. Therefore it is to personal preference of every participant to go for safety or style. So there is NO BAN but also NO OBLIGATION for helmets. You might probably find modern helmets with vintage look.
A road or racing bicycle is ridden wearing a matching jersey. Up to the early 1980ies these have been made from wool only, later (category 5) first funny synthetic jerseys appeared. Jerseys are matched by black cycling shorts, short white socks and lace-up shoes (for category 5 you might wear early velcro shoes ... in neon colors ;-)) PLEASE DO NOT wear jogging or trekking shoes on a road bike!