In D&D 5th edition, the protection fighting style is a single defensive maneuver which is not as versatile or effective as other fighting styles when it comes to mounted combat. It’s really considered subpar because its best use seems to be on those rare occasions when you find yourself dismounted. More often than not, a player will find more technical and tactical value from the battlemaster fighter.
The game designers haven’t given much attention to the type of combat most heavily employed by feudal knights, namely hand-to-hand combat on horseback. Mounted knights had very different weapons and tactics from foot soldiers, and were absolutely at home in mobility since nobility spent most of their lives riding across battlefields with enemies all around tryin tog kill them.
The protection fighting style is a perfect example of how D&D 5E does not emphasize mounted combat. In fact, the only time you’re going to get any use from this maneuver is when you find yourself without a mount, or when you’ve been dismounted. The description states that the aim of the maneuver is to attempt to deflect an incoming attack. It effects the next melee weapon or spell attack that targets you, but does not state if it can be used against ranged attacks in any way, shape or form.