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.
Not currently, no. We support Debian Jessie and Debian Stretch. The reason for this is that the core functionality of the VM is managed with Puppet code from rOPUP Wikimedia Puppet. The Wikimedia Puppet codebase currently supports Ubuntu Trusty, and Debian Jessie & Stretch. Wikimedia's main production network is phasing out all use of Ubuntu in favor of Debian. This has been an ongoing project for several years now. A more modern Ubuntu distribution may be supported in the future depending on the outcome of T169099: Locate an alternative source for modern Openstack Debian packages.
I believe that it is a good decision to move just to Debian rather than support Debian and Ubuntu. Since Ubuntu is build upon Debian anyways.