I have been around a lot of good/great bowlers in my life, and have never met one that made up/back adjustments on the approach unless it was to get around the ball return.
Could anyone provide an example of someone who is above league bowler status that actually use these adjustments this century?
I've bowled with some good guys who do it, regional players (I've never bowled with pros nor asked them). It's not as common as it once was, and like me it's usually not the first choice but I've seen very good tourney guys move (generally up).
I am not wanting to get in a pissing match, but honestly in a tournament setting with a lot of pros, you'd have to be extremely motivated to pay attention to where everyone starts and when they adjust and compare. Have you actually done that? Who announces "Hey everyone I'm leaving 10's so here I am moving 6" back!"??
When I make a forward/backward move (which isn't often), I just do it and no one around me is seeing it. They'll notice other overt changes, but how the hell could anyone keep up with the precise start spot of a house full of A-grade bowlers throughout a whole event?
Would it validate me if I said "Norm Duke does it", nor change your mind about its effectiveness? Likely not, and if not a single pro ever changed starting positions, I'd still know from doing it that I've had good luck with it on the rare occasions it's what changed the pocket for the better.