Reaper's Harvest crate items will remain exclusive to that crate season. Whether that crate season will be reissued for a limited time isn't known yet.
Several Witches Festival seasonal limited crown store items will likely return, albeit maybe not in the exact order they were released in prior years.
A few of the 4.1.0 datamined items may replace older Witches Festival seasonal items, and a few will be exclusive to the Hollowjack crate season. As we're still a few months off, the team is probably still finalizing which items will go where.
Wraith of Crows is not a seasonal polymorph. Both skeleton polymorphs are earned in-game.
Thank you for the insightful response! I was hoping someone a little more observant than me could help me plan ahead. I didn't realize the skeleton was earned in-game. To Google!
Edit: I have the skeleton polymorph now. Haha. It's so good. If anyone else is looking for it, head to Stillrise Village in the upper left corner of Shadowfen.