This strikes me as the PERFECT kind of topic for the "Modularity" that is being toted for the new edition.
There ought not be any "rule" for magic items, but a set of "options" to be chosen from by the DM/group (or make their own!) for their preference of playstyle.
I think the suggestion for a general guide (low magic=3 items, "average" magic=5, high magic=7) is a perfect example for any game.
Add to that the full on "Slots" option, for those that like/want it. With the proposed guidelines that you could do: head, neck, back, body [front/torso], belt/waist, arms, wrists, hands, feet, 2 rings...and/or anything in between.
The "No slots" option...either because you're following the "3/5/7" rule, because you're not going to have magic items at all OR you ant to use...
...the "Go Gonzo" option!

(for those that like/want that) Wear 2 rings on one hand, 3 rings on the other and put one in your nose? No problem. "Yes, your leiderhosen +3 works just fine with your socks of endless odor and the boots of levitation." You find a headband of intellect, a hat of disguise and earrings of comprehending languages? All yours, man!
Additional optional things to point out for the DM:
1) Magic Items all work together just fine.
2) Some/Certain items may cancel each other out ("Your Flaming sword seems to flicker and snuff when you put on the Breast Plate of the Frost Giant King") or do wonky things to their effects ("Your flaming sword dims a bit, but is still burning. But your new breast plate seems to be endlessly dripping/'leaking' very cold water, leaving a noticeable trail on the ground")...or YOU! (a.k.a. DM's FUN DAY!

)
3) Items do not/will not work in tandem/stack, so you really have to keep to slots to make sure everything works as it says on the box.
4) Items do not/will not work in the same vicinity! "Aura crash!" You get ONE magic item on your person...and best to keep a minimum 10' from the other items in the group or nobody's +'ing anything!
5) Make it an element of Class...non-caster's can effectively get/use X items of Z type without repercussion. Casters can possess up to Y of any type, but can only use 1 at a time...
6) What-/however you want.
Anywho, I trust the point is made, use of magic item "Slots" as a "rule" in the books not only seems unnecessary, but at odds with the "optional/modularity" proposed for the addition. Let the DMs/groups "Choose Your Own" (gods, I loved those books

) way to incorporate them.
Have fun and good morning, ENworld!
--Steel Dragons