Not exactly so. It is available to PCs of deities where it 'fits'. Some good deities have been specifically excluded, eg Lucinda, as She allows necromancy and necromancers amongst her order, something that wouldn't sit particularly comfortably with an Undead Slayer.
However, it it fits for your character and it fits for the deity, then yes it would be considered. It is not only for clerics and paladins of Toran and Aeridin, it is just historically worshippers of these deities have accounted for the majority of applications (as it suits Their dogma perhaps best). There have been others, Cymeran Vrinn for one springs to mind who iirc was a cleric of Az'atta.
