Much of that seems to be hype from a US competitor because in practice the form of phenidone used in Ilford commercial developers is/was stable, some now use Dimezone since manufacture was sub-contracted.
Originally Posted by Rudeofus
Some books claim Phenidone has a poor shelf life (as a raw chemical) but I was using Phenidone made in 1962 until my supply ran out 4-5 years ago and it was just as good as my newer supplies. It's often forgotten that Phenidone is a Trade name and there's a number of variants, there were issues with the original form of Phenidone but by the time Ilford marketd Phenidone based devlopers these had been overcome.
In practice the ID-78 I make up lasts well over 2 years in a good plastic bottle, it's the warm tone variant of ID-62, but shelf lifes no different to ID-67.