The Season 15 premiere was actually packed with nods to earlier hunts, including all the way back to the pilot with the Woman in White and Sam saying “We got work to do” over the trunk of the Impala.
Fans couldn’t have possibly guessed back in the early days of Supernatural that the final arc of the series would be about Sam and Dean trying to defeat God with their angel pal and kinda sorta grown-up Nephilim kid, but those earlier seasons evidently could come in handy for recognizing Easter eggs in the final episodes.