Long ago, the Wikimedia Operations team made the decision to phase out use of Ubuntu servers in favor of Debian. It's a long, slow process that is still ongoing, but in production Trusty is running on an ever-shrinking minority of our servers.
As Trusty becomes more of an odd duck in production, it grows harder to support in Cloud Services as well. Right now we have no planned timeline for phasing out Trusty instances (there are 238 of them!) but in anticipation of that phase-out we've now disabled creation of new Trusty VMs.
This is an extremely minor technical change (the base image is still there, just marked as 'private' in OpenStack Glance). Existing Trusty VMs are unaffected by this change, as are present ToolForge workflows.
Even though any new Trusty images represent additional technical debt, The WMCS team anticipates that there will still be occasional, niche requirements for Trusty (for example when testing behavior of those few remaining production Trusty instances, or to support software that's not yet packaged on Debian). These requests will be handled via phabricator requests and a bit of commandline magic.