Let me start by saying I don’t think glare is a horrible thing in every game. In COD Warzone for example, I think sniper glare is a necessary mechanic. The map is so big that the glare is a nice way of balancing it out, otherwise it’d be far too easy to just snipe from a concealed position.
In Halo though, the maps are nowhere near that size. Even on the biggest maps we have right now, if a sniper can see you, chances are you can see them too.
More importantly though, sniper weapons already have built-in mechanics to stop them from being too OP. The sniper leaves a vapor trail, the stalker rifle fires bright red shots, and the shock rifle has shining electric bolts. All of these things make it easy to find out where a sniper if firing from. I personally think adding glare on top of this is just too much of a handicap for long-range weapons.
Could just be me, though. What do you all think?
EDIT: I forgot to mention that the player outline system also makes it very easy to spot enemies.