There's no way to determine exactly. But I used one as soon as Gen VI BH and PH was in place. But I can safely say I have seen ~5 ate Shedinja right from the beginning of Gen VI. Maybe someone else faced a lot of them.
I peaked at #9 on the BH ladder, mostly owing to my Dual Shedinja (One ate and one Sturdy) and of course, Regigigas and Chansey. Fill in the other two spots with Offensive Powerhouses (No setup) and you're good to go.
lol. i peaked #1 with a dual shedinja team not too long ago, one with aerialate(personally better then the other two since the rare heatran exists and i prefered killing it, then potentially killing ph gira) dual shedinja has always been a thing with scrappy assisting, allthough now with the 30% power boost on espeed it actually does a decent amount.
also:
To be honest, the main issue with the whole idea is Rocky Helmet. Some of my best teams lack a Ghost-type, relying entirely on Rocky Helmets and various forms of passive damage to kill Shedinja. Now obviously this isn't the best idea, but -ate Shedinja still loses to all of these things, so if I were to switch in, say, Rocky Helmet Fur Coat Darmanitan-Zen, it would die regardless.
However, I have struggled to counter these things in the past, but it's probably a good idea to include things on your team that serve as both Sturdy and gimmick Shedinja counters.
rocky helmit doesnt solve anything for the main problem with shedinja. in that its a pivot with revenge killing properties. now most people are oddly into the revenge killing side, but in my personal experience just having shedinja able to hop in and switch out on powerful attacks feared me WAY more then endevoir EVER could. in fact ive never lost to a endevoir strategy before...but i HAVE lost because shedinja effectively pp stalled my offensive threats with baton pass. forcing a forfeit on my end. personally, i am fond of running a pursuiter with priority(mega mewtwo x/y work) whilst having them run rocky helmit(making them 100% shed counters) however, moldy gengar still takes the cake.
another thing is scrapsheds main purpose is to ko something
and to make sheds life easier, if it hits a rocky helmet user, then all shed has to do is clean it up with ice shard meanwhile if it somehow gets healing, you at least know it has rocky helmet, so you dont endevoir stupidly, honestly, i found it funny because people kept trying to eliminate my scrap shed, moreso then my REAL shed, while my scrapshed was actually my least valued member of the team(half the time it did nothing. i even considered removing it at one point but kept it anyways).
but honestly, for dual shed, the best method of action is just attacking even if you know sheds coming out, i mean, eventually they have to make a risky play with scrapshed, and once that sash is gone switch to your shed counter, since they wont want to take another powerful attack. i mean, when i used my dual shed team, i swear NO ONE actually tried to outpredict me. and i always won due to it. it was kinda sad. and if they set up rocks, and one of my 3 defoggers just removed it right in their face.
thats another thing i want to throw out there, triple defoggers is actually really good, worth it for any shedinja team, my contrary team had it, and i swear shedinja never died, i had a bouncefogger, contrary offensivefogger, and prankfogger while always getting mocked for using it(and prank xern ;~;), my contraryspam team actually got pretty popular due to its effectiveness and got me pretty high up on the ladder(i want to say 1, but i think i fell short to 3 or 5 idr.)
