This article is a bit late—season 9 of Adventurers League play is set to start in September—but I wanted to note, for the sake of completeness on the topic of the "PHB +1" rule, one major exception: The Embers of the Last War story season.
The Wayfinder's Guide to Eberron
In July 2018, Wizards of the Coast announced the Wayfinder's Guide to Eberron (WGtE), a 5th Edition sourcebook PDF for the Eberron campaign setting. WGtE is considered unofficial playtest content, and a number of items from the guide were released as Unearthed Arcana articles:
The announcement of WGtE was followed up shortly by Encounters in Sharn, a collection of short side quests and vignettes meant to flesh out campaigns set in Sharn, the City of Towers.
Embers of the Last War
The announcement of the WGtE was unexpected, but more surprising was the announcement of Embers of the Last War. Embers of the Last War was a new story season, running concurrently with the Waterdeep story season, that consists of twelve adventures, taking PCs from level 0 (see below) to level 10. It also made Encounters in Sharn an AL-legal adventure, to allow DMs to give PCs the opportunity to gain levels to keep up with the recommended levels of the adventures.
Embers of the Last War is notable for three things:
It's the first AL campaign not set in the Forgotten Realms. Characters from other seasons can't play in Embers of the Last War, and characters from Embers of the Last War can't play any AL-legal content other than the adventures specified in the AL Content Catalog for the story season. (People could argue that the Curse of Strahd story season is the first campaign not set in the Forgotten Realms, but the PCs trapped in Ravenloft were still from Faerun and returned there if they were able to escape the Demiplane of Dread.) DM Rewards, and DM Quest rewards earned in earlier seasons could not be used on Eberron characters, and rewards for running Embers of the Last War adventures could not be used on non-Eberron characters.
Players have unrestricted use of unofficial playtest material. The "Awakened Mind" certificate allowed the bearer to create a PC using the Unearthed Arcana article for the Mystic, but the content from WGtE was available to all players.
It's the first campaign since the Curse of Strahd story season to not use the "PHB +1" rule. The League released a separate Adventurers League Player's Guide (ALPG) and Adventurers League DM's Guide (ALDMG) for the story season. The ALPG allowed players to create characters using the Player's Handbook, Xanathar's Guide to Everything, the goblinoid races from Volo's Guide to Monsters, and the WGtE. An early draft of the ALPG did not include the Bugbear, Goblin, and Hobgoblin races but players pointed out that Darguun is a nation of goblinoids in Eberron, so they were added to the final version.
What's a "level 0" character?
The first adventure, What's Past is Prologue, set up the backstory to the storyline. Players used one of five "level 0" characters (pregenerated characters with race and background features, but no class features or abilities) to play through the adventure. The AL rewards earned for player are assigned to a new level 1 PC. Which brings us to...
The future of Eberron in the Adventurers League: Oracle of War
Earlier this month, Wizards of the Coast announced Eberron: Rising From The Last War. This hardcover is the official 5th Edition sourcebook for Eberron: it will provide the final versions of the character options from the WGtE, the final version of the Artificer class, and a mini-campaign for players now to the campaign setting. Wizards of the Coast have stated that they will update the WGtE with the final versions of the playtest content and include the Artificer, but other content from the new hardcover will not be added to the PDF.
As with the announcement of the WGtE, the League announced a second Eberron story season, Oracle of War. Details of what content will be available to players for Oracle of War are forthcoming; when the ALPG and ALDMG for the new Eberron story season (and for season 9 of the Forgotten Realms) are available, I will revisit this topic again.