Once we have a PHP 8.1-flavored mw-debug deployment (T372604), extend x-wikimedia-debug-routing.lua [0] to support it as a backend.
This is a pretty straightforward change, but needs to wait until we've chosen a service name and port.
Once that's done (and we've validated manually that it works as expected) we'll also need to update debug.json [1] in order for the extension to offer it as an option.