I couldn’t agree more on the topic of mandatory training for supervisors (and not because we’ve had that here for as long as I remember): it provides a basic level of assurance and support to new supervisors of our postgraduate researchers.
(The full report also points to the value of team supervision and the time pressure supervisors are feeling which I could also comment on at length!)
https://wonkhe.com/wonk-corner/theres-new-research-on-how-pgr-students-are-supervised/