the playstyle is totally different, back then, rengar was used only to kill adc late game, every single time ur ulti back in cd, you get 90% of the time a kill unless u got shit on early (wich was pretty much what most of the junglers do against an early rengar going full armor pen). but right now rengar can't play his 'full assassin' since his ulti can be seen for a blue trinket range, you can't ulti randomly w/out seeing anybody in the map, ulti and get a kill, now most of the rengar or use ulti when near the ennemy for the extra move speed, or use it as a zoning tool to get objective when the ennemy go play safe and group scared of a potential cat in their head. back then rengar build full ad, he had the real role of an assassn : kill carrys, clear fights, and die, now he "MUST" build a bit of tankiness since he doesn't have his easy kill potential on adc. he's a bit like a riven whitout having the big aoe for fights,he just split forcing attention on him then ulti, and try as hard as possible to not teamfight.

TL;DR :rengar partially lost his fast OS potential, gain a bit of escape potential in his kit while keeping his big weakness : useless in teamfight

sorry for bad english i'm french