World of Warcraft Classic takes a heavy toll on the player's free time so some less than legal shortcuts were bound to happen. Some players figured out a way to farm dungeons' end bosses and now they might get banned for it.
World of Warcraft Classic is mostly as the game was back in the mid-2000s, at launch. Back in that time, players were fine with taking long treks without mounts, five-hour dungeon lockouts and tons of grind for the sake of levelling to 60. Now some players have figured out how to bypass the lockouts and even better - completely skip all the dungeon enemies up until the final boss.
If you are getting a feeling that this sounds too good to be true, you are right. The method was not a legal one and was a result of an exploit. In the official forum post addressing this problem, Blizzard didn't specifically state how the exploit worked but they did note that a hotfix should put a stop to it.
The company didn't stop there though and announced that those who abused the exploit will face "punitive measures" which probably means those players are going to get banned. Keep in mind that if you happened to participate in such a "run" you may still not be eligible for these punitive measures.
Blizzard's statement specifically said the company will look into account activity and identify those "who knowingly abused this bug in an exploitative manner" which more than likely means it's only the repeat offenders who will get the ban hammer. Therefore, if you got invited into a group, killed the final boss and left, chances are the devs will treat your account as if it was a normal case of joining a party that was on the last boss.
The date and time of hotfix deployment had already passed at the moment of writing this article so it is probably safe to join any groups that are on the last boss, if you feel up to it.