"Jerry Zhang via GitGitGadget" [off-list ref] writes:

Internal usage of patch-id in rebase / cherry-pick doesn't persist
patch-ids, so there's no need to specifically invoke the unstable variant.

This allows the unstable logic to be cleaned up.

While all of that may be true, two things are not explained.  

 * Why does "unstable" need to be "cleaned up"?  Is that too dirty
   in what way?

 * If internal usage does not persist patch-ids generated by the
   machinery, why is it bad to be using the unstable variant?  A
   naïve expectation would be to make sure you use stable one if you
   want a future recomputation to give you the same result, but the
   opposite does not have to be always true.