As far as I've heard - it is still an individual store policy.
Many stores do not allow it because the really don't have the space and access for overnight parking.
Many stores have dropped allowing overnight parking because of some people thinking it was a free campground. Setting up chairs and the BBQ, staying for several days, etc.
Some cities such as where I live, have modified their zoning to not allow overnight occupied parking (sleeping in the rig or truck) in store parking lots.
I've seen people with extension cords run to jury rigged outlets in parking lot light poles. I've seen people with their grey water draining out of a water hose onto the parking lot/ ground/ storm drains. I've seen people leave bags of trash in the middle of the parking lot as they left.
People used to treat an overnight stop at a Walmart as a thank you from the store for RVers patronage.
Many people today treat an overnight stop at a Walmart today as their 'right' do use the parking lot any way they wish.