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.

Does Constantine show up in Lucifer?

During his brief cameo appearance on The CW's Arrowverse crossover event "Crisis on Infinite Earths," it was revealed that Tom Ellis' Lucifer Morningstar actually had history with Matt Ryan's John Constantine.

What episode does Constantine appear in Lucifer?

The Flash 6x09 "Constantine Meets Lucifer" Season 6 Episode 9 HD "Crisis On Infinite Earths Part 3"