I have an old Shimano RD-M760 DEORE XT Rear Derailleur, where shifter cable cut a deep groove into the guiding steel tube. Here is a picture:
Is that tube replaceable? I think that groove is a contributing factor to too much friction in the cable system, which makes downshifting sluggish or skipping gears.
Here is a wide angle picture to make my description more clear. The shifter is in the lowest gear (largest sprocket):
and a closeup with adjusting barrel and the cable removed:
The tube with a groove cut into it, is not a back end of the adjusting barrel, it is a separate part. I don't see a way to easily remove it, but it does rotate in place. Maybe I should rotate it 180 degrees, so the cable doesn't sink into the groove anymore. OTOH, the groove is so smooth and nicely shaped as if it is a part of the design. Is it, though?
It seems that the groove is not a part of the original design. Here is a picture of an old Shimano RD-M760 from Ebay, showing just a slight indentation on this part nowhere near the depth I have on mine: