Rengar is still one-shotting everyone in less than a second...

And his own ultimate counters him

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He was always intended to be able to, he's an Assassin, and one without an escape.

But he does have counterplay, in the form of Pink Wards and Red Trinket, which destroy his engage potential.

Pink wards and UPGRADED sweeper do not counter him. Even if spotted while in his ult it doesn't stop the extra damage and leap attack. Yeah you can see him coming but the best times to gank are when your cc and escapes are already down. It's like seeing a fed Master Q come at you while hearing Ghostblade and his ult pop. You might land a snare but unless it's a Morgana snare he'll catch you it'll just take a second or two longer.