i dont see how it could be device specific, any device supported by ios 9 should have it, all it does is drop wifi connection when it thinks its too weak, in favor of using cellular for data so the user is not stuck sitting there with a minimal wifi signal until ios timesout the wifi & switch to cellular. ios already always had the wifi signal limitation, there was or still is, a jailbreak tweak to enable ios to detect & stay on all wifi signals until it is truely too weak, not just at apple's setting.
i tried it & dont really like it, my router w/ddwrt is good & handles weak connections well, sometimes with wifi assist on i see the wifi get dropped for cellular when its at 1 & sometimes 2 bars..even at a solid 1 wifi bar i get over 4mbps of solid stable low latency throughput, enough for HD content, but ios thinks thats too weak & cuts it so i run with the setting disabled.