Here's the synopsis for Supernatural season 15 episode 16.
 
FLASHBACK TO A YOUNG SAM AND DEAN – Sam (Jared Padalecki) and Dean (Jensen Ackles) are asked to investigate the murder of a childhood friend, calling them back to a motel from their past and perplexing the brothers with a case they thought was solved a long time ago. Amyn Kaderali directed the episode written by Meghan Fitzmartin (#1516). Original airdate 10/22/2020.

Source:

CW

 
A final flashback episode? We will see young Sam and Dean one more time. Who is the childhood friend who was killed? How do they find out the murder wasn't really solved? Give us your thoughts in the comments or in the discussion thread for episode 16.