Red Dead Redemption 2 was clearly designed as a third person game, but you don’t have to play it that way if you don’t want to. You can play the game in first person view as well and when you do that you can also shoot from the aim down sight view, by using the d-pad.
If you want to go into first person mode for a really immersive experience you can do so by changing the camera by pressing the back button. This will cycle through a couple of alternate views as well. Once you’re in first person mode, you can then aim down sights when you have a weapon drawn by pressing down on the d-pad.
Shooting does feel a bit more natural with being to aim down sights in first person. Aim Down Sights in First Person Mode is a toggle switch. You will need to hold the LT/L2 button to aim your weapon and then once aiming you can press up to look down the sights of the weapon. This will be your default view when shooting until you toggle back.
While using ADS in Red Dead Redemption 2 you can still use all the features of third person, which includes Dead Eye and its different capabilities.