The start page lists what was released in the last 7 days and what's coming, by date: new Pokémon, shinies, Shadows, Dynamax… with any event starting that day.
Release dates come from the Pokémon GO Wiki and Leek Duck, checked daily, and are reviewed by the RepDex admins.
Upcoming on a card: announced with a date, not out yet. It shows greyed with a green border and isn't counted until its date.
Unreleased: not in Pokémon GO yet, no date. Shown greyed for completeness, not counted.
Battle-only forms (like Zen mode Darmanitan) are shown but can't be ticked.