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....
I barely even noticed they got replaced. They never got to have any story involvement or character development, just like most of the rest of the bridge crew in Discovery.
I really found the coda/epilogue to be distracting. The end of the main story ended in a good place, with Burnham and Booker heading off on their next adventure. And then suddenly we're getting this long drawn-out "and then later after many adventures they... did something useless we can't explain but at least they saw their old friends again the end" just really took the wind out of it.
Why Detmer & Owosekun Were Missing From Star Trek: Discovery Season 5 Explained By Showrunner ( screenrant.com )
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....
Inside the ‘Star Trek: Discovery’ Series Finale: The Last-Minute Coda, the Surprise Easter Eggs, and What Season 6 Would Have Been About (EXCLUSIVE) ( variety.com )