Auto manufacturers are required by US Federal Law to have parts available for any models they sold for the period of 10 years.
I have also heard this claim from various people over the years, with numbers ranging from 5 years to 20. It seems reasonable, and was stated in one of the Snopes threads linked above, and implied in the mechanics.se answer (specifying California law), that manufacturers would need to provide parts necessary to fulfill their requirements under warranty. I can especially see the feds regulating availability of parts related to federal emissions and safety standards.
On the flip side an unresolved warranty claim is purely a civil matter, so the need for such a federal law eludes me. Additionally, a manufacturer most certainly would not want to irritate their customers by telling them to junk a brand new major financial investment because they do not have brake pads available. This logic makes me skeptical that a federal law (or at least that a broad 'all parts' law) would exist.
So is there or is there not a Federal law or regulation? If so, is this law/regulation for ALL parts, or just a subset of parts? References? I realize that it is almost impossible to prove the non-existence of something, but I would consider an answer to be legitimate if such a law used to exist but no longer does (as that would at least fortify the claim).