Note that the eternal serpent frame can still be received. (Various contests online and probably next irl events when they’ll be able to)
Crown of purity seems linked to charity event so you can also guess (but not confirmed) than it can go back with others charity events.
And to be fair, this is something that they should make for every ‘limited frame’, cycling them back at various points for various events (so we could get a christmas event for example even with old frames :p)
Yeah, the Eternal Serpent frame can be earned through forum contests (I’ve won one of those myself), but the initial way you got it (through the Warhammer Fest) was not revealed before the event, and serves as an example of Fatshark not being all that good at announcing time-limited collectibles.
When it comes to the Crown of Purity, I kind of doubt it would be given as a reward for another charity event. People that already got the hat would not be compelled to donate, since they already have that cosmetic. And that would hurt the event’s cause. If another charity event happens, my bet would be on Fatshark creating a whole new cosmetic, just for that event.
As you know, I disagree that time-limited collectibles should be made available again, and from the looks of things, Fatshark seems to be of the same mind. Apart from the Fangtastic Fortitude and Eternal Serpent frames, no time-limited collectible, has been made available again.
2020 is over. In Happy New 2021, happiness should be available for everyone. At least in Vermintide.