I'm not sure the issue is entirely a matter of phenidone's toxicity; I think it's at least partly because phenidone appears in such small quantities. A typical PC developer has about 0.1g of phenidone per liter of working solution, or proportionally more in a concentrate. I've seen many MSDSes that include notes to the effect that substances at under a 1% concentration are omitted. There's about ten times as much metol in a typical MQ developer, by comparison.Phenidone is so little toxic that it need not be mentioned in MSDS sheets.
That said, my understanding is that phenidone is among the least toxic of the common B&W developing agents.