They COULD film a cameos in the last to EPs with actors off set and cut it in....they did that with Adrianne Palicki in the Season 2 episode "What is and/What Should Never Be" . She was not available to shoot the last scenes where Dean decides to break out of the dream so they filmed her somewhere else (she was on another show then) and edited her in later.
Comment on Supernatural coming back: this episode was filmed long ago!! The ONLY episodes that were filmed AFTER quarantine were the final two 15.19 and 15.20!! So this "Last Holiday" episode was meant as a mid season light episode.
They COULD film a cameos in the last to EPs with actors off set and cut it in....they did that with Adrianne Palicki in the Season 2 episode "What is and/What Should Never Be" . She was not available to shoot the last scenes where Dean decides to break out of the dream so they filmed her somewhere else (she was on another show then) and edited her in later.