I don't know. Is there really a signal here when it comes to humans? It seems most of the arguments come from one study? Would be interesting to hear those here at HN that are smarter than me comment on this one.
There's also an emerging literature suggesting that deficits in specific amino acids are a kind of mammalian signal to start burning fat, which might also be related. Cysteine is one, which might also explain why plant-based diets are healthier all other things being equal, because plant proteins tend to be lower relatively in cysteine. So it might not be proteins in general whose restriction results in longevity increases, just cysteine-rich proteins.
For example: