I’ve looked in the DMG and the PHB, but have only found ancillary uses of the shield. The Shield Master feat (PHB, p.170) gives you neat perks, but doesn’t act as a weapon (beyond the bonus shove action).
Does shield proficiency automatically grant you shield bashing?
What would reasonable damage be?
And do the old rules apply for losing the defense bonus when using it as a weapon?
Does it count as a light weapon? Can I use it as a two-weapon bonus action?