That tweet linked in the starting post is not entirely accurate. Only cosmetics that appeared in any given seasonal loot box will be unlockable with Overwatch in-game credits. Past play-to-earn cosmetics and other limited-time items are not included for this event.

Yes, all seasonal event items as long as it can appear in any loot box from a seasonal event.

We will be force-opening any of your collected loot boxes on September 14th. This will give you several days to decide what remaining cosmetics you want to unlock with your Overwatch in-game currency before we launch Overwatch 2.