Obi Wan is gung-ho on killing Vader throughout the entire trilogy. Especially in this movie, where he says the Emperor has already won since Luke won’t kill his own father. He seems to imply that was always the plan for Luke, too. Which means Vader’s line suggesting that Obi-Wan once told him to run away together contradicts itself within its own film. So I seriously hope this line is amended. Even ignoring the prequels, it makes zero sense based on what we know about the characters.
First, Vader says “Obi-Wan once thought as you do”. Originally Obi-Wan was gonna try and bring Vader back to the light after he turned, but failed. He’s referring to a past attempt.
Second, Lucas didn’t intend on it coming across that Obi-Wan was gun-ho on killing Vader. Obi-Wan saying that Luke not being able to kill his father means “the Emperor has already won” is simply him acknowledging that if Vader refuses to turn and puts Luke in a position where he has to kill him and he refuses, then yeah, the Emperor will win.
“He knows that, eventually, Darth Vader is gonna come looking for them. He knows this thing is going to blow up in a big war. He knows a confrontation is brewing between Luke and his father. Ben hopes that Luke will either save his father or kill him, because whatever extra powers Luke’s got in his lineage, he is the one person that can probably fight his father and win.” - The Star Wars Archives: 1977-1983, 2018
Now, obviously Obi-Wan isn’t optimistic that Vader can be saved. He does think Luke will be forced to kill him, hence why the dialogue is the way it is.
Regardless of if it came across well in the film itself, the Jedi don’t explicitly send him to murder Vader, they just tell him to confront/face him. And we also circle back to my first point.
I’m indifferent about the line change. I think the original line works because I view the originals as a separate entity from the prequels as is (even if I still enjoy ROTS to a degree) and the new line works because Vader acknowledging Luke’s mother is rather touching. I don’t think it’s a make-or-break type of deal.