Actually, the filename is "fm_01_gang_leadership.pdf". fm_01 -> Fanatic Magazine 01. The keywords "fanatic magazine" are a clue, too. The layout style is also a giveaway, but only if you already know about the magazine, I suppose.
Outlanders is the supplement box for the original Necromunda, produced a couple of months after the original boxed game was released - among other things, it had rules for Outlaws, Ratskin Renegades, Spyrers, Scavvies and Redemptionists.
Fanatic Magazine was the main method of support for Fanatic Games, so no, it wasn't "produced after GW dropped support of Mordheim". In fact, GW ending support of Mordheim would be when they stopped publishing Fanatic Magazine. It's not as obvious as White Dwarf coverage, but then only Mordheim and Gorkamorka really got significant additional rules published in White Dwarf. For the most part, it was the individual magazines from Fanatic Games / Specialist Games, and then Fanatic Magazine that added the additional material. They were all sold in stores, too.
Technically the original rules didn't cover the loss of a gang leader, but most people had the gumption to come up with the obvious solution. The addition of leadership challenges was a new thing, but I like it. For one thing, it adds story potential for named hired guns wandering the underhive looking for revenge, a second chance at glory and another bottle of Second Best.
It was never really that common - you need to have a Leadership higher than the gang leader's for one thing. Assuming a standard gang leader and a standard ganger, you need two Ld upgrades - a 1/144 chance - and then there's a 1/3 chance that he won't challenge anyway. If he does challenge and wins, there's a 1/3 chance that the leader sticks around. There's a 4/9 chance, overall, that a leadership challenge results in the gang leader leaving the gang. If it really worries you, play Redemptionists.