On the 5-seater version, it should be like this:
YouTube
See at 0:33 - 0:39.
The 7-seater has the rubber on the 3-rd row seat backs, why wouldn't it be the same for the 5-seater? Beyond me!
My needs aren't so much something to trap water or dirt. I need protective, smooth rubber to slide things in and out - with the seats down too. I had a simple such mat for the rear of a Land Rover Defender, OEM; fit great and did the job. It was just for the floor but the material was perfect for this purpose.
As I also have a tray in my 4Runner, things are complicated further. I RARELY use the tray but would like access to the rear "cubby". I would still buy the 5-seater version with the accordion and seat back covers, cut it where it flexes over the cubby and apply velcro strips under it to adhere it on and off. A simply tear off from the velcro and you see the cubby.
I know, too much what if for my case, and those with the tray. Plus the accordion style isn't available for the 5-seater. I think the only way this would ever materialize is for a DIY job. Buy the material and cut to fit it yourself. Three pieces for the seat backs and one for the floor. Plus an accordion strip, two. A generous application of velcron strips along the seems will hold everything in place.
Or, do the seat backs yourself and buy a ready made one for the floor.
The key will be material choice. You want something that's not too flexible so the accordion will not stay semi-rigid horizontally. What would work for me is the plastic material used under office chairs to protect the floor. Some are even made with nubs or teeth. Invert these, so the nubs are up to grip things placed on the floor, and cut to shape. Or, if a smooth surface doesn't bother you, leave the nubs down.
I know, this would be a pain to cut to shape, with all the cutouts involved. But, material cost is minimal if you want to put in the effort. The material is also easy to cut, unlike rubber.
Just a thought. I'll probably just leave it at that and just throw a plastic blanket back there when I need it. Like I do now!