Deployments don't always have keep-around refs
In 8.9, deployments didn't always have keep-around refs. This means that we can have an environment created that refers to a deleted ref, which didn't cause a problem until 8.11, when we started showing this info on the MR page. When someone tries to load any MR on that repo from then, it will fail with an exception because it can't find the ref.
This is causing this customer issue:
ZD: https://gitlab.zendesk.com/agent/tickets/39674
/cc: @MrChrisW