In Star Trek: Discovery season 5, episode 5, “Mirrors”, Detmer and Owosekun were assigned to pilot the Mirror Universe’s ISS Enterprise to Federation HQ by Captain Michael Burnham (Sonequa Martin-Green). This explained Detmer and Owo’s absence for the rest of Discovery season 5.
However, Detmer and Owo do appear in Star Trek: Discovery’s epilogue scene, and they were a sight for sore eyes.
That explains the story they used to explain in universe, but not why they chose not to have them on the final season.
@Hobbes Click through to the story, where Michelle Paradise says “They had other projects during that time so we got them when we could.”