With Cylc 7 the cylc validate command used to list unused task definitions when run with the -v argument.

This was an obscure, but potentially useful feature for those wanting to tidy up the runtime section. These tasks were logged at the WARNING level.

At Cylc 8 as part of cleaning up command output, removing spurious warnings and obscure options, this warning was removed. This doesn't really make sense as a warning as it is common for workflows to turn tasks on/off in the graph with Jinja2 switches but leave the runtime behind. Another use case is defining extra tasks (e.g. diagnostic tasks) which aren't in the graph but may be inserted/triggered manually when required.

The idea is that warnings are things users are supposed to take action on, we've made an effort to reduce "arbitrary" warnings and output in general al allow users to focus on the important information. As such I don't think that this functionality should be implemented as a warning in cylc validate as it was before, however, this functionality might fit neatly into cylc list.