Tippett may still get programmed - Edward Gardner has been doing so, for example - and could enjoy a resurgence of interest soon. I agree that PMD has been very poorly served by the recording industry. I'm not sure why as from reading these pages he seems to have a lot of supporters.I'd say so. But it's a very subjective idea. I tend towards the wider definition and would include Michael Tippett for example, and Peter Maxwell Davies (no complete recordings of his symphonies and is rarely programmed at concerts).
Neglected is a difficult concept. Much of, say, Berlioz or Dvorak are also rarely programmed but are known as among the greats.