Fireflash is a down->up invincible reversal, so extremely easy to do and not possible to cross up the inputs for it (which is a common way to beat DP’s). Maya’s original leap kick was only vulnerable for like 1 frame, and as a QCB DP that could be made safe on block (or that could fly so far away that it was nigh impossible to punish) was suuuuper abusable. Glay’s heavy puddle punch is invincible and also a quarter circle motion, and in my opinion he’s now the character most likely to wake up with DP if you’re standing beside him (his original puddle punch wasn’t truly invincible). Kan-Ra’s original sandsplosion is another good example of a truly degenerate invincible reversal - it had an insane hitbox, was often unpunishable on block, and only required a single button press.
You’d be surprised. A lot of the godlike hit confirms you see at super high level are the result of a strong buffer or OS game, as opposed to incredible reactions per se (though the reactions are definitely a part of it). Many top players are buffering supers when they toss out buttons period, so the only part they need to “confirm” is just the final button press to activate the CA. Other times, it’s just a straight up OS, where they’re supering, but it won’t come out unless the button hits something.
Characters that can alter their jump trajectories (Sadira, Hisako, Cinder) are pretty adept at this. Pretty sure I did it at KIWC against Thompxson, and Kalypso is very solid at doing it with Sadie.