Despite being the one race in the galaxy that was actually accustomed to living their lives aboard spaceships, the quarians were not included in Mass Effect: Andromeda. The reason, presumably, was so that EA could make a big post-release splash by adding them, along with some of the lesser-known species, in an expansion, something it teased in an Apex Mission in May.
Whether that was actually the plan was never confirmed, but any hope that it was fell apart in August when BioWare announced that no further "in-game story content" would be released. But story content released outside of the game is a different matter, and so it is that the tale of the quarian journey to a new galaxy will be told in a novel written by Hugo and Nebula Award winner Catherynne M. Valente called Mass Effect: Annihilation.
"This official tie-in novel chronicles the journey of the Keelah Si’yah as it carries 20,000 drell, elcor, batarian, and quarian colonists to Andromeda," BioWare said. "A pathogen is discovered aboard the ark after many drell are found dead in their cryo pods. As the pathogen jumps species, the ship’s systems begin to fail, making it clear this is no accident."
The Keelah Si'yah, according to the Mass Effect Wiki, departed for Andromeda after the human, asari, turian, and salarian arks, because of delays caused by the more complex biological needs of its inhabitants.
Based on the responses to the news on Twitter, not every Mass Effect fan is happy with the novel-instead-of-an-expansion wrap-up to Andromeda. But it's not a hasty fill-in to tie up loose ends, nor is it even actually new: Annihilation was revealed in December 2016 as one of a trilogy of Mass Effect novels, which also includes Nexus Uprising by Jason M. Hough and K.C. Alexander, which was released in March, and Initiation by N.K. Jemesin and Mac Walters, slated to come out on November 28.
The only thing that's changed as far as I can tell is the release date: Annihilation was originally scheduled to come out in the summer of 2017, between Nexus Uprising and Initiation, but was delayed—possibly to accommodate the decision to drop planned Andromeda expansions.