There is an adventure from the 3rd season of Dungeons and Dragons Expeditions called DDEX03-07 Herald of the Moon. It awards a very rare longbow as the main quest reward, called Shadowsong. Shadowsong is a special version of a more general magic item, called an Oathbow. Here is the description of Shadowsong, from the Rewards section of the adventure PDF:
Weapon (longbow), very rare (requires attunement by a ranger)
Each of the elven oathbows are possessed of mythical power and ancient legends. Shadowsong is hewn from a supple length of yew and features curious green metal tendrils snaking through its length. It glows dimly in the presence of humans. The description of this item can be found in the Dungeon Master's Guide.
This bow is a bit odd, as it can only be used properly by rangers. This is not the most convenient attribute, but it also makes it a bit more interesting and special. However, Wizards seems to have decided that this is a problem, and issued guidance in the season 9 content catalog (5 years after the release of the adventure) saying that this item is inappropriate for Adventurer's League play, and should be replaced with a +2 long or shortbow instead.
This adventure features an item that is no longer acceptable under D&D Adventurers League guidelines, as it deviates from the attunement standards in the DMG. Instead of Shadowsong, this adventure instead unlocks a +2 weapon (longbow or shortbow). This +2 weapon has the sentinel property and glows dimly in the presence of humans.
One of my players noticed this after we had already run this adventure, and after some characters had been reworked to multiclass into ranger to allow them to use it. Although some players were happy enough with the change, others were rather frustrated, as it seems to be compulsory to make the switch.
However, the explanation for this guidance doesn't seem to make sense. How does this bow deviate from the attunement standards in the DMG? The Dungeon Master's Guide explicitly mentions that an item which requires attunement can have a class as a prerequisite, on page 136:
Some magic items require a creature to form a bond with them before their magical properties can be used. This bond is called attunement, and certain items have a prerequisite for it. If the prerequisite is a class, a creature must be a member of that class to attune to the item.
So what's the problem?