The lingual frenulum is rarely portrayed in art.
Hendrick de Keyser (1565-1621), a Dutch sculptor and architect, created a spectacular one (see below) in 1615, it forms one of the lines radiating out from the protester's nose.
Here's a bust Keyser made in 1615,
but the best-known portrayal of this neglected ligament was made by the extraordinary Franz Xaver Messerschmidt (1736-1783) in 1770;
It was after Messerschmidt's death that someone named this piece “The Yawner”, an appellation that seems to have stuck. Messerschmidt didn't name it anything. He didn't name any of the forty-odd “character heads” that he made after he had been forced from public life (he was thought by his aristocratic employers to be mad). Giving such a name to this piece seems reductive, there is far more than a yawn going on here, assuming there is a yawn involved at all. I suspect we keep our sublingual ligaments hidden when yawning...
Self portrait (circa 1783)
… a pandiculating Joseph Ducreux certainly did. We should pay attention to Ducreux, he was an exact contemporary of Messerschmidt
Self portrait (1893)
… and there wasn't much you could get past him.