There is no prohibition in the manual for sunglasses.
My opinions, for what they are worth:
Traditionally, sunglasses are not something that should be worn. However, they were not traditionally worn in baseball for more than a century, but look at what we have now. If you are on the east side of a N-S field for an early evening kickoff with clear skies, it would seem reasonable to wear sunglasses while the sun is above the artificial horizon. Once it gets down below the top of the grandstands, take them off. No reason for anybody else to wear them.