The League was a Gym Leader-like system where the site's best battlers took on the responsibility of being a Gym Leader. People would challenge them, and once they beat all or enough, could challenge the very best of the site to become champion or a Leader themselves.
That's the basic version of how it worked, but unlike most crappy showdown leagues we actually port time, effort and organisation into it.
The league was created 6 (maybe 7?) years ago and then rebooted 2-3 years later to reflect the new generation of games and new wave of users.
Being a Gym Leader was a big thing back then. You had to be voted and approved by multiple people so you had to actually be good, and similarly meant that actually collecting several badges also proved you were good as well. Essentially, it was a real, actual test of skill. The league worked by assigning people to smogon tiers and metagames, meaning that there was a heavy focus on being well-rounded. Being well-rounded versus opponenets that heavily specialised was difficult, and thus collecting more than one badge was genuinely difficult.
The League died due to a lack of interest and the significant people involved leaving. Most of the people who wanted to be in the League either became Gym Leaders or worked behind the scenes, meaning that there weren't a lot of actual challengers coming through. Everyone got bored. Leagues are fun but not sustainable. You need a very large but not too large community to handle one, as getting too big would sort of make them irrelevant anyway as better tournament structures handled greater numbers of people.
As for who was involved - a whole bunch of people ranging from Mods like Mike, trachy and Mew to lowly (not even Expert at the time) little ol' me (fun fact, I was the Gym Leader for OU and was chosen over the previous Gym Leader DoctorFlame, who was more or less objectively better than me at OU. People can make dumb descions). There was only two Champions, Hex and Mewderator.
