Warning: SPOILERS for Lucifer season 6 and Legends of Tomorrow season 6.
The final season of Lucifer failed to answer a question regarding the larger Arrowverse; what was the favor involving Mazikeen (Lesley-Ann Brandt) that John Constantine (Matt Ryan) and Lucifer Morningstar (Tom Ellis) worked out in some off-screen encounter? This question was raised by Lucifer's cameo in the Crisis on Infinite Earths crossover event, but has only served to raise more questions in the minds of fans. It seems these questions will go unanswered, with Lucifer having come to a close and John Constantine having been retired from the Arrowverse.
The unexpected meeting between the two Vertigo Comics mainstays occurred during the third part of Crisis on Infinite Earths, when John Constantine, John Diggle (David Ramsey) and Mia Smoak (Katherine McNamara) traveled to Earth-666 to seek Lucifer Morningstar's help in opening a pathway to Purgatory. The brief scene, set sometime before the start of the Lucifer series, established that Lucifer was part of the Arrowverse and that Lucifer Morningstar had worked with warlock John Constantine in the past. Though clearly annoyed by the intrusion into his Earth, the Devil always made good on his deals and Lucifer agreed to help Constantine, noting that he was only doing so because "I owe you for Maze."
The brief scene was widely agreed to be one of the best moments of Crisis on Infinite Earths and fans hoped to see another encounter between Constantine and Lucifer at some point in the future, either in Lucifer or Legends of Tomorrow. Matt Ryan and Tom Ellis were both reportedly agreeable to the idea and writer Marc Bernardin pitched a crossover special that would answer the question of what Constantine did to merit a favor from Lucifer. Sadly, the special never occurred and Constantine does not make a cameo in Lucifer season 6.
Tom Ellis also did not appear as Lucifer Morningstar in Legends of Tomorrow season 6, which turned out to be Matt Ryan's last turn as John Constantine, possibly due to production starting on an HBO Max Constantine reboot produced by J.J. Abrams. The Legends of Tomorrow season 6 finale, "The Fungus Amongus," (which aired the same week as Lucifer season 6's premiere) reset the status quo for John Constantine on Earth-Prime, with the Laughing Magician betraying the Legends as part of a bid to regain his lost powers by tapping the mystic Fountain of Imperium. Constantine's actions nearly destroyed the Fountain and he survived the experience only by reselling his soul (which he had regained during Legends of Tomorrow season 5) to "an old associate in Hell." The episode ended with Constantine breaking off his romance with Zari Tarazi (Tala Ashe) and leaving the Legends to "walk a different path."
While the reference to "an old associate in Hell" might be taken as a nod to Lucifer Morningstar, this theory ignores that the Devil of Earth-666 didn't traffic in souls. It also neglects to consider how, in the cosmology of Lucifer, the only mortals who go to Hell believe they deserve to be punished for their sins in life. It also flies in the face of how Lucifer changes Hell from a place of torture into a place of healing in the final episode of Lucifer. All of this fails to address the plot hole of the different versions of Hell in the Arrowverse, creating yet another question to be ignored, along with what John Constantine did for Maze to merit a favor from Lucifer.