The team first announced Midnight Depths in September. They did so with the belief that the update would release soon-ish. However, it has been delayed multiple times. The team provides an update to the community in this regard,
“In the interest of making sure we release the best content possible for this finale to our first Serial Story, we're making the difficult decision to push this content back until Early 2021. When it launches, you'll have an opportunity to face off against Graknash once and for all, and finally learn the secrets of the prison on the moon.”
The team didn’t cite the ongoing pandemic as the cause here either, as has been the case for several other games releasing in this most unique of years. Additionally, they aren’t citing time for extra polish as has been the case for Cyberpunk 2077’s multiple delays. Whatever the reason, we’ll just have to wait a little longer to play The Midnight Depths.