Let's take a quick look at the Patch 4.20 notes for preseason, the patch where they removed Feral Flare and the Spirit Stone/Quill Coat item paths...
This preseason, rather than trying to kick the best champions back in line, we instead focused on taking bigger steps to bring strategic diversity back to the jungle. If we can build a jungle that empowers and reacts to the many different actions junglers can take, then we can create an ecosystem where it's almost impossible to distill an "optimal" jungler at any point. That's a bit of a muddy concept, so we'll explain: in the 2014 season, the jungle was a fairly 'solved' position, where optimal routes were established, sustain was easy to come by, and monsters were just piles of gold waiting to be picked up. Because of this, the 'best' junglers were quickly discovered to be those who could heavily influence lanes while still keeping up in their farm; in other words, gank / damage focused champs (oh, hello Lee) who don't make many tradeoffs for their superior dueling strengths.
For a period of almost three years, jungle difficulty was trivialized. You would go machete 5 pots (read: 750 bonus HP) and if you could make it to 700g for your spirit stone, you could technically jungle. On the surface, this seems like a hotbed for diversity, making nearly every champion in the game capable of jungling. However, what wound up being the end case can be summed up in this statement: "If I can always survive the jungle no matter who I play, I need to play the champion who can exert the most map pressure through ganks that still makes it through the ban phase." If you watched professional play in this ecosystem, every game invariably wound up being either 
vs. 
or 
vs. 
If you ask me, the Spirit Stone items hit the right tones when they enabled every champion to access the jungle. The problem comes about when you make a jungle where your only basis for comparison is gank strength. My suggested change is to introduce even more jungle enchants to the game so that an even wider variety of champions have access to the jungle, while keeping sustain from being frontloaded past the point of no return.
TL;DR, The Spirit Stone items were enjoyable, but their mere existence discouraged diversity, requiring them to be removed. Their theme of sustain can easily be recycled if you tone back their numbers as a meaningful tradeoff for sustain.
is a champion that has historically only worked and enjoyed popularity at Gold and below. From a G&B perspective, seeing a lot of Master Yi doesn't necessarily mean that Master Yi is a strong champion who is a preferred pick, it means that players don't always know how to play against him. Historically, players below don't know how to abuse a jungler with a weak early game.