Shimano Derailleur seized - but not the clutch

You'll need to remove the jockey wheel cage. There will be dirt or corrosion on the pivot and bush.

Bit of a light sanding, grease and refit will do it.

NSMB.COM "https://nsmb.com/articles/shimano-derailleur-clutch-service/"

Shimano Derailleur Clutch Service

Did you know Shimano's clutch mechanism is serviceable? This service could improve your drivetrain's performance and longevity, which is particularly important right now considering the global shortage of bike parts

Main pivot behind the clutch is the likely culprit. Clean out and reassemble with waterproof grease. You don’t need to take the mech off the bike though, just pay attention to how you wind the mech cage.

As above.

This is a 12sp but the same method applies. I had one that was totally seized and couldn't be removed. Hopefully yours isn't that bad!

Magic - the GMBN video is what I was looking for.

Thanks.

My vote is that TJ slathered it in loctite

Six year old M8000 rear mech here. Never had that problem - but I strip it down to its components and rebuild it once a year, as per the many YouTube vids on the subject. Do that and you'll be reet. It's easy enough once you've followed somebody else's tutorial through once or twice.

I think this was the tutorial I first followed for rebuilding an 11 speed mech. Worth doing even just once to understand how the rear mech functions.