Locked bones of proxies should not be editable, at all. But lack of
update from linked rest pose in 2.7 allows to pose and animate locked
bones (not to pose them without animation though, or you'd lose your
pose on next file save & reload).
this is used by artists to always lock all their bones in a rig, so that
proxies fully update when lib rig is modified...
For now, restore that broken behavior in 2.8 by not updating proxies
against lib armature in CoW context (makes sense anyway, we are
currently doing a lot of useless thing when copying data for depsgraph
evaluation!).
f0c0d11ea2