Deck Your Collection (Season 2)

I can confirm that the achievement, and therefore the shirt, is indeed still obtainable as of 7.3.5, as I got mine today after days of farming random brawls.

A few things to keep in mind, if you decide to go for this achievement:

Even if you have a shirt from a previous season, it doesn’t necessarily count towards this season’s achievement. You have to kill the bosses required in the achievement, using the random brawl system, started by using the [item=143763] , bought for 100 [currency=1299] from [npc=115797] (Horde) or [npc=118898] (Alliance).

It is true, that if there is a boss, from which you do not have the shirt, the odds of you getting queued up against that boss will be very high. However… If you, like me, got some of the shirts back in MoP and WoD, the odds won’t be changed as you “already have the shirt”, and since there are seventy-something bosses, it might therefore take you a quite a while to beat the ones you need. It took me a solid 3 days of farming random brawls to finally queue up against [npc=70666] (Who I had previously killed in MoP and therefore had the shirt he drops), which was the last shirt-dropping boss I needed this season.

As LimeXLight and others pointed out, there is no way to find the achievement, unless you’ve already earned it in a previous season, since it has been added to legacy achievements. You can however track your progress by using the script:
/script AddTrackedAchievement(“9177”)
There is no way to control what boss you queue up against though, so there is nothing to do but queue until you get the achievement - Though it’s always good to know how much you’re missing.

Link showing achievement, mail containing shirt and shirt (Yes, it still says “… during the Draenor campaign” even though it’s done in legion - Not sure if it still being obtainable is intended or not, but here we go): https://imgur.com/T7CMEXI

Best of luck getting the shirt, hope this helped!

There seems to be a lot of conflicting information about whether this achievement is still doable in BfA, and if so/not, under what circumstances. Me being the stubborn achievement chaser that I am, I had to find out for certain. I can now confirm that I received the achievement AND the [item=98543] via mailbox, last night (9-22-20). You can view the screenshot here.

As has been stated, there is no way to track your progress other than manually. I use the add-on "All The Things", which is the only way I even knew about the possibility of collecting the shirt. There really wasn't much point in using the macro to add it to the objective tracker, since you can only see the first 5 bosses in the list, nor does it check them off as you kill them. All the bosses required for the achievement are listed in ATT, so I just used that instead. As others have also stated, it turned out Master Boom Boom was the final boss I needed, and apparently rarer in the randomization algorithm. It took me about 3 days, 2-3 hours per day, and many, many, many boss repeats before I finally got him in random battles.

To set the context of my character's interaction with Brawler's Guild to date, this same character earned the Brawler's Guild invitation achievement on 3/2/2013, and earned the achievement to win a brawl during the Pandaria campaign on 6/1/2013. And that was the last time I entered the Brawler's guild until this month. So I don't believe there are any pre-requisites or past achievements needed to acquire this one. I've never attempted Brawler's Guild on any other character on my account.

I've successfully done and gotten credit for all of these brawls in season 3 via the Random Brawl option... except for Master Boom Boom. I'm beginning to think that brawl is bugged, as there is some random brawl which is bugged and will immediately boot you out of the ring instead of spawning the boss.