Yes. They Can. Newsom is doing it under the whole "Green Energy" conservation movement going on there. He has the receipts to enact it too with all the electricity issues Cali has had in the past and while it may not be in effect it has been passed through the legislature so it is a foregone conclusion that it will be enforced very soon.Cali is still draft such mandates. It is not in effect as of today. Cali cannot just banned outdoor grows a good percentage of the licenses on record are outdoor grows. Just not sure how many are using them these days
I also base this on a company I used to work withs actions. They own one of the largest indoor facilities in the West located in Needles California. They bought truckloads of double ended fixtures to install in their bloom rooms before they started construction. About a year ago they sold off all the DE's and converted all their lighting to LED due to the changes being enacted. I don't think a company that large would take a loss that large if they weren't sure that this is where it is going to go.
The outdoor ban is also being held up but will eventually be enacted and enforced. AFAIU, it only pertains to open grow cultivators. Green house/Light Deprivation ops are exempted. Green house is designated as a structure with at least 8 feet of solid walls. No hoop houses allowed.
This is what was relayed to me through my weed attorneys Bruce and Allison Margolin. That may have changed recently but that is what I understand it to be.