It was only fitting that with the end of Feud: Bette and Joan that Ryan Murphy showed the end of Joan Crawford's acting career. While Bette Davis continued to work until her death — albeit with lower standards, as Dominic Burgess' Victor Buono states — Crawford's acting career was finished years before her death. The Feud season finale indicates that it was a combination of things that led to her career coming to an end, but that one of the major factors was her role in the movie Trog. While Trog wasn't actually Joan Crawford's final role as Feud shows in the April 23 finale, it was indicative of how far her she had fallen in regards to her status as a leading lady.
As The Doors' "The End" plays in the background of the episode "You Mean All This Time We Could Have Been Friends?" Jessica Lange's Crawford wanders around the set of Trog in a daze. The song choice couldn't have been any more appropriate as her career had officially deteriorated by this point, which was the reason she starred in this sci-fi B movie about a caveman who is alive in the present day. After filming Trog, Feud has Crawford outraged at the signing of her book, My Way of Life, and disgusted with the photos of herself in the paper at an event for Rosalind Russell. The TV series shows that this was the end of her time in the public life.
While it is true that Crawford retired after the event for Russell, that occurred in 1974 and Trog was released in 1970. Between the B horror film and her retirement, Crawford did have at least one TV role. According to her IMDb page, her last on-screen performance was a guest spot in the TV series The Sixth Sense in 1972. She portrayed a woman named Joan (how appropriate) in the episode, "Dear Joan: We're Going to Scare You to Death."
Wikipedia has some other assorted TV spots for Crawford, but besides narrating a documentary or showing up in a revised version of something she already filmed, The Sixth Sense (sans Haley Joel Osment) was the end of her acting career.
Yet, it makes sense that Feud decided to streamline the finale episode and just show Trog as Crawford's last role since it's much more symbolic or her descent. In Feud, she takes the role of Dr. Brockton in Trog in part because she always wanted to portray a scientist, in particular Marie Curie. Crawford was an early film star whose career had managed to go from silent films to talkies — but, as Feud shows it, Hollywood didn't want her once she had aged. Although she had dabbled in B movies even before What Ever Happened to Baby Jane?, her career after the 1962 movie was pretty much full of only B movies or TV spots. It all culminated with the absurd Trog.
In some ways, Crawford's performance in Trog is admirable as she is actually decent in the face of otherwise ridiculous moments. And thankfully, the internet has provided Crawford fans with plenty of clips to watch her final film role.
For both Crawford and Davis, their last few performances were not an indication of the illustrious careers they had led before then. As Pauline said in the beginning of the Feud: Bette and Joan finale, Crawford seemed "very much tossed away" and Trog, in all of its B-movie glory, epitomizes how little Crawford was valued in her final days as an actress.