For any technical issues you may be experiencing, please visit the official tech forum here: https://forum.deadbydaylight.com/en/categories/technical-issues
All purchasable content of Stranger Things will be removed, if you got anything before the date and time, you will keep it.
If you are not able to obtain it beforehand, they'll be gone for good after the 17th November.
Here's a FAQ to everything: https://forum.deadbydaylight.com/en/kb/articles/292-f-a-q-stranger-things-last-chance-sale