Comment 1 for bug 1851489

The removal of cleanup logic was a deliberate step to ensure that no Juju infrastructure can self-destruct under any circumstances.

As a compromise, machine initialisation has been modified to add an executable script to /sbin for cleaning a controller or machine previously provisioned by Juju.

So there is still a manual step required, but it is simply "/sbin/remove-juju-services".

The patch is here: https://github.com/juju/juju/pull/10877