What happened to Feral Flare?

There are two main reasons to this.

Mental aspect
Feral flare has a quite bad reputation right now. Mainly because people often associate it with the "Afkfarming Jungler" since it requires you to kill jungle camps to gain extra stacks (or even the item itself). Then again this reputation doesn't come out of nowhere. At the release of the item a lot of junglers just went up farming for 30 minutes straight and while yes, they were strong after these 30 minutes, their team often lost due to the fact that they were basically playing a 4v5.

The Stats and gold efficiency
Let's face it... Feral flare has horrible stats for most junglers. The item itself is worth 74% of its value and the passive requires you to kill a big monster for 1 extra damage. That means at best if you are only afkfarming in the jungle you might get 3 extra damage per minute and you also need to be farming all day to compensate for the gold income. Also the 10 health on return are not really great. The only thing it's really good for are the high-range ward and the faster clearing of objectives like Baron, Dragon or buffs.

Now I'm not saying that it is a horrible item per se but all the other jungle items are a lot better. Elder Lizard provides you with more Damage through his AD + True damage and Ancient Golem gives you a ton of tankiness and on top of that both items will provide you with an insane amount of HP and Mana sustain.

The nerfs Hit feral flare really hard mainly because you need more stacks and because they nerfed the Attackspeed on the item. Xin-Zhao is literally the only champion where you could still consider buying it if you just look at the stats but then again: You'd have to farm a lot to benefit from the item and this is almost impossible since Xin is among the slowest junglers in the game.

It's better to just go for either Golem or Lizard on these champions while ganking alot instead of buying an item that is useless if you're not afk farming

Because it's not very gold-efficient, and its not really that good in most builds, its very specialized.

That said, it's still broken on some junglers imo. Warwick and Xin come immediately to mind- BotRK, Feral Flare, and they're already virtually unkillable (At least before you die, in Xin's case. He doesn't have that much sustain.), never mind before tank items.

So really it's not hated in most cases, people don't use it because it's a very specialized build for it to be worthwhile.

You can't. It's simply not viable now. You need at least two items (generally feral+BoTRK) to be effective. If you just go feral then full tank, you're not taking advantage of the item at all, and you're trading your first 15+ minutes to do it.

A good discussion can be found here.