How did people unbox tiered cosmetics from 2015 and onwards with Scream Fortress 2013 effects?

[iron1024x1024]

Stuff like the Warhood and Rotation Sensation with Hellfire or Darkblaze, how something like that is achieved? As far as I know, only till 2017 it could be done, since no 2018 cosmetics have Scream Fortress 2013 effects that I have found. And also supposedly only the Spooky Crate could give those effects, and can't be unboxed anymore.
Was it some sort of glitch? I'm absolutely baffled!

[ᴾᴾᴹ Shuffle Spy]

https://wiki.teamfortress.com/wiki/Scream_Fortress_2015 - This brought back the prior Halloween effects for 1 event

 

Edit: It was 2017, not 2015

[Cedarium]

2 hours ago, ᴾᴾᴹ Shuffle Spy said:

https://wiki.teamfortress.com/wiki/Scream_Fortress_2015 - This brought back the prior Halloween effects for 1 event

I think it was SF 2017, not 2015

[Seminal Inhalation]

1 hour ago, Cedarium said:

I think it was SF 2017, not 2015

Yup
https://wiki.teamfortress.com/wiki/Scream_Fortress_2017