We have now support in the puppet tree for building Debian Buster based proxy servers in Toolforge (related: T235059: Toolforge: refresh puppet code for proxy (dynamicproxy) to support Debian Buster)
Currently, tools-proxy-03 and tools-proxy-04 are running Debian Jessie, so they need to be rebuild and switched from the old role (role::toollabs::proxy) to the new one (role::wmcs::toolforge::proxy),
The work for doing such rebuild has been scheduled for 2019-10-28 14:30 UTC. An announcement for the operation was published already: https://lists.wikimedia.org/pipermail/cloud-announce/2019-October/000226.html.
A checklist and concrete operation steps will be added to this task previous to the operation window.