I agree with this feature request and am surprised it doesn't exist already. Here's my similar write up of the request:
When using the Power Query editor I would like to see an additional option in the Steps right-click menu to allow me to Comment or Uncomment a step. When debugging I often create filters and I'd like to be able to easily turn them on or off. Commenting a step would use the same logic as your Delete command by updating any referencing steps to refer to the step above, but instead of removing the step it would prefix it with //.
You could make the prefix include another character like "//!" so it could discern between commented steps and regular comments. Commented steps with the special prefix could still show in the Steps panel but with distinct formatting (green font, italics, etc.). The developer could right-click on a commented step to restore it. The assumption would have to be that any references to the step above would now point to the restored step.
Vote for this if you think it would save you time troubleshooting your queries.
