Note to self: Clean/re-grease derailleur pivots periodically.
Kinda surprised me this as the derailleur isn't that old as such and doesn't show any bad play in the other pivots. I've a theory that I might have used a thin oil like GT85/TF2 down there at some point and flushed out the grease, and somehow water/mud found it's way in past the seal and ground it to death. I think it could be worthwhile repacking the grease periodically (not that they're mega expensive to replace, but seems a waste when other than this the derailleur is fine).
Just noticed this recently as the gears started skipping towards the top of the cassette, and even jumped over into the spokes at one point (not good for spinnergy wheels! ).
A lesson learnt (for me) is all.
Mtbr's 2016 Winter Biking GearReviews and Roundups
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Thread: Shimano Derailleur Pivot Wear