is rageblade still a good buy for jhin?

In theory? Sure.

In the actual Game?... Not really... Why? Because the you simply dont need what it offers. And you cannot make use of it before a normal game ends.

I'd consider it as 5th Item.

The bonus AD from Rageblade and Jhin's passive used to stack on each other infinitely, which gave him a much higher AD than normal

So now it's still a decent item, but not as overloaded as it was before on him

Personally I don't think so. He no longer gets ludicrous amounts of AD from its passive that compensated fro his lack of synergy with on hits or very slow stacking of it. Now that it gives normal amounts of ad to him I think you are better just getting criticized or penetration.