Middle Fork Trail is open.
You are right that the website lists the status as closed but if you call the Lytle Creek Ranger Station 909-382-2851 they can confirm that it is open. You will need a online permit which can take 3 days to get to hike into the Cucamonga Wilderness which is pass the wilderness sign that is about 2 miles from trailhead. Note that the dirt road is very rough and has deep potholes in places which will require a 4×4 vehicle. Also there no overnight camping currently in the forest.