Actually, it is still there, in
https://kindlemodding.org/jailbreaking/WinterBreak/ :
"If an “Unexpected error” occurs when you try to log in to the Kindle Store or only the Kindle Store home page is displayed, try the following solution:
1 - Factory Reset Kindle"
At this point the Kindle have chances to likely be Jailbroken already by potential attempts of running Winterbreak.