Idk for sure how buffering shield drops work, but as far as I can tell, it doesn't truly buffer, at least not in the same way as C-sticking rolls, jumps, and dodges. Based only on how shield drops feel, they occur between 40-50 degrees. Idk how precise the stick even is for shield tilting, and idk if that chart ajp once posted even included shield tilt angles because as I recall his chart had fairly large gaps between the cardinal directions and other angles, but I feel like you can tilt a shield very slightly off of cardinal directions. I also don't know how going to the side plays a role in the shield drop window. Simple logic would dictate that the further to the side you press your stick, the more sensitive it is, making it easier to distinguish between the area where you will shield drop and the area where you won't. But if the stick processes angles the same way regardless of how far from the center it's pressed, then I don't see how that would matter outside of it just being easier to hold fully to the side.
ANYWAY, shield drops don't have a permanent buffer. I know this for sure because I can shield drop with the stick at a certain angle and drop, but if I just hold it at that same angle during shield stun, I won't (I first noticed this during moves like Falcon's knee or stomp which have a LOT of hitlag and stun). BUT, if I only hold it at that angle for the last few frames of shield stun, it does seem to buffer. So from that, I've come up with two possible explanations:
1. The game actually does have a small buffer for shield drops. I know it does for FFing out of hitlag (you can input a FF 2-3 [?] frames before hitlag ends and still get the FF). This means that you CAN hold at a single angle and buffer the shield drop, but you'd have to time it within a few frames.
2. If the first possibility is false, then shield drops seem to buffer because there is an actual window within which you can input a shield drop as opposed to a single point. If you can only drop through by holding at the 45 (which is how I've always taught people to think about it), then it's extremely unlikely that I have superb timing of hitlag and am able to consistently hit that angle right when shield stun ends. More likely, all angles from 40-50 or over some other window all initiate a shield drop. If this were the case, then even if shield drops didn't actually buffer you could simulate a buffer by rotating through multiple degrees as shieldstun ended. As soon as it ends, you'd be continuing your rotation through lower degrees and would shield drop asap.
I have no actual evidence other than my experience, but I doubt #2 is the case. While it's entirely possible shield dropping has a window larger than just 45 degrees, I have to believe there is some type of buffer because I often swear I am keeping the stick steady at 45 and still drop through on frame 1 (and again, I'm not timing it that perfectly). I'd like to do more research with Yoshi's shield to see how hitlag affects things because I know Yoshi's shield doesn't undergo hitlag. I'm wondering if Yoshi can actually shield drop DURING the opponent's hitlag, but NOT DURING the shield stun only a few frames later. I might just be misunderstanding how Yoshi's shield works though because that seems really weird and random.