Greetings!
The foundation has long since stopped using Debian Stretch for new
projects, and will start actively removing it from the data centers this
summer.
In the meantime, I don't want any one who is on the fence to choose
Stretch when they could choose Buster, so I'm removing it by default
from the image options in Horizon. If you still need to use it for
legacy reasons, feel free to contact a cloud admin or open a phabricator
ticket; restoring access to a specific project is simple. I'm also
leaving it enabled for the deployment-prep and integration projects as
I've no doubt they'll still need to create Stretch VMs until it's fully
phased out in production.
-Andrew