That's still used in many places, but it does not resolve anymore (internally nor externally), there's still ptr records pointing to it too:
03:39 PM ~/Work/wikimedia/operations-puppet (production|✔) dcaro@vulcanus$ dig +short -x 185.15.56.18 instance-mx-out01.cloudinfra.wmflabs.org. mx-out01.wmflabs.org. 03:39 PM ~/Work/wikimedia/operations-puppet (production|✔) dcaro@vulcanus$ dig +short mx-out01.wmflabs.org
For now the other record that resolves internally was manually configured to get unstuck, on the vm:
dcaro@accounts-appserver5:~$ sudo grep mx /etc/exim4/exim4.conf route_list = * mx-out01.cloudinfra.eqiad1.wikimedia.cloud:mx-out02.cloudinfra.eqiad1.wikimedia.cloud # route_list = * mx-out01.wmflabs.org:mx-out02.wmflabs.org