The thing is, if it was done right, you could still have Anikin fall and Vader appear in the PT without ruining ESB’s reveal. And if you were watching the films for the first time, ESB’s reveal would make you realize what really happened in the PT, making both moments even stronger.
The PT should seem to line up with what Ben tells Luke in Star Wars. That Vader murdered Anikin.
Ok, but since Vader is Anakin there is no way to tell this story without showing that. You could have Anakin falling in a lava pitt, and then suddendly appears a dude called Vader in a suit, who is supposed to have been Ben’s pupil and the murderer of Anakin. The all idea is dumb (Q2 tried it in his fanedits, it does not work), you can’t “do it right”. No one in the world with a fonctionning brain wouldn’t immediately think “hey, that guy Ben is talking about, it’s Anakin! Why is the screenwriter trying to make a secret of something so obvious ?” and ruin ESB reveal with an entire movie ahead anyway. And that’s the all point of the PT. At least, one among others. Now (I-VI order), the real reveal is: how is Luke going to learn the truth ?
The only thing that could have been kept secret, is Leia being Luke’s sister (Q2 tried it in his fanedits, it kinda works).
edit: nevertheless, GL could have kept the “suited” Vader final look a reveal for the overture of ANH. Anakin could have been last seen is ROTS in some kind of proto suit, an unfinished armor, or just his body lying on the medical center table being fixed (no black cape, no mask).