The created rig seems to miss some copy rotation constraint that should be set on foot_heel.ik:

Copy Rot 1
sub target: foot_heel_ik.L
Order: Default
use_X, use_Y, use_Z
invert.X
mix_mode: Before Original
target space: Local Space
owner space: Local Space

Copy Rot 2
sub target: foot_ik.L
Orger: Default
use_X, use_Y, use_Z
invert.X
mix_mode: Add
target space: Local Space
owner space: Local Space

This should retain a reverse rotation from foot control.
Except for this issue the sample is generating correctly

This breaks backwards compatibility with previous uses of the rear_paw rig type. This is because the old rear_paw was just substituting in the regular paw, which requires a chain of 4 bones. This new implementation demands a chain of 5 bones. Fixing this backwards compatibility might be possible with some kind of versioning? All that would need to be done is switch any existing uses of the rear_paw rig type to the regular paw rig type. I have no experience with versioning code so I'm not sure if this is possible or a good idea.

About backward compatibility, in current rigify there's no option in "super_limb" for "rear_paw", it just runs te paw code, so older rigs should just run the code with old 4-boned metarigs and regular paw code. The rear paw sample was created for consistency and as a placeholder for possible new sample (which is the case we are working on).

The ideal case is just the ui showing a new "rear_paw" subtype under "super_limb" subtypes list. I'll provide updated metarigs corresponding to new 5-bones setup to be included along as the patch.

Should be all done I think.

if @Alexander Gavrilov (angavrilov) has no objection on code side, i'd accept the patch