Zed is still strong, he can still blow you up. i have a friend right now who streams, is master tier, and exclusively is playing zed to reach masters again on his smurf. he can still take over games with the right builds and set up for a late game carry.
the reason people think he falls off is because they build him entirely for his early game power with just bork into ghostblade > lw and then go into regular items or defensive items after that. not to mention zed like any assassin with a delayed damage source that can be cleansed will be counterable by a lot of things. there are tons of champs who counter his ultimate, (kayle, zilean, tryndamere, kindred) and you can also buy items (zhonyas, QSS). So you gotta build him to actually deal insane dps late game. my bud has been going standard zed (bork ghostblade start) into Infinity Edge/triforce(or PD depending on game) and it works really well. he also brings exhaust so they have to choose whether to cleanse his ult or the ensuing exhaust. it leaves Zed relevant late game.
Zed is still a good pick imo and in a very healthy state. if you buff him, he will become oppressive in lane and just instantly delete people with little to no counterplay. keep in mind that zed is so strong, that you have no choice but to counterbuild against him or risk him destroying you. If you are a mage and you don't build armguard one slip up means death. and that is STILL the case. look at faker vs febiven in this year's MSI.
TLDR= zed is great still, just not favored in current meta.