In amateur costumes, leather armbands are very common. Some imitate metal, but most are brown faux-leather vambraces that sit above the wrist.
There is no historical basis for this trope. I wonder, how did it begin?
Was Quo Vadis (1951) the first to use leather armbands? 1934's Cleopatra has metal bands on male characters.
Does this go back to 19th century plays about antiquity? When did these armbands come about?
What counterexamples do we have which demonstrate that leather bracers were never used by the Romans? The answer to that question might have some places for us to look for the beginning of this trope.
Thomas Dudley Fosbroke, in his book Encyclopædia of Antiquities, makes mention of of competitors wearing wooden bracers to protect their forearms during play.
Here is a depiction of Roman gladiators wearing precisely this type of armband/vambrace. Article isn't terribly good, but mentions that the piece is/will be on display at the Villa Giulia. I'll admit I haven't seen it in person, so I'm open to someone explaining that it's non-authentic if you know more. Various mosaics display arm wrappings/armour for gladiators, although often (not always) in the form of the larger manica worn by the murmillo.
I don't find the Augustus statue counterexample terribly compelling, since it should not be viewed as a catalog of all (including optional) pieces of armor and accoutrements available.
It's not a myth. During the Imperii they used to cover the arm holding the weapon.
It's shown in some effigies, such as the ones made for the Tropaeum Traiani.
