Not sure what you mean, since all diadem mats can be purchased pretty cheaply with yellow scrips. The Pegasus mount and minion will presumably still be gotten somehow or other since it wasn't tied strictly to the old Diadem gameplay anyways. It will either be gotten through gathering instead of battle Spoils or dumped into some other content like treasure maps/dungeons or scrip purchase or ventures or whatnot. It's only the battle content and achievements tied to said content that we know is definitely being removed. Gathering content will still exist in whatever new form Restoration Diadem takes, and any gathering-related rewards or items that you could get will most likely still be available in the new Diadem, or elsewhere.
EDIT: Just noticed that the highest-tier mats aren't available for purchase. Stuff like the peteinosaur skin and such. But again, they will almost definitely add those to the scrip purchase list like they did all the older mats, or include them in the new diadem gathering content. There's no way they would simply remove all those mats AND all the gearsets in the crafting list that use them. Of course I would never have believed they'd flat out remove the achievements either....but here we are.