I checked my Toyota's manual and it does explicitly warn about neutral for long periods of time for this reason. However, when not in neutral, the car will not allow the traction battery to be below 40% or so, so it is difficult to enter a car wash with an empty battery.

However, the car may have a very small, old, or weak battery; there may be significant drain on the battery (air conditioning?) while in neutral; and you may be in a very long car wash.