The Explorer's Guide to Wildemount is official in that it is published by Wizards of the Coast, but none of its content is permitted in AL play. This announcement on the D&D AL website says:
As awesome as Critical Role is, we must say a Mighty Nein (that means “no” in Zemnian to those unfamiliar with the setting) to travelling to Wildemount in your Adventurers League campaigns. There are no plans to allow Explorer[']s Guide to Wildemount or any of its offerings in the way of spells, classes, feats, etc. as a valid resource for Forgotten Realms or Oracle of War campaigns.
Everything below is in a sense obsolete, as there is no more PHB+1 rule, but the new structure for AL sources still does not permit EGtW.
Furthermore, the AL Player's Guide explicitly lists what sources are permitted by the "PHB+1" rule in the Forgotten Realms AL campaign:
Forgotten Realms characters can choose race and class options from the
Player’s Handbook and one other resource—a rule called “PHB+1.”
Additional resources include the following products:
- Elemental Evil Player’s Companion (EEPC)
- Sword Coast Adventurer’s Guide (SCAG)
- Volo’s Guide to Monsters (VGM)
- Xanathar’s Guide to Everything (XGE)1
- Mordenkainen’s Tome of Foes (ToF)2
- Locathah Rising (LR)
Additionally, when selecting an additional source, the following
variant or optional rules are available when you create your
- Variant Human Traits (PHB)
- Half-Elf and Tiefling Variants (SCAG/ToF)
- Option: Human Languages (SCAG)3
- Blessing of Corellon (ToF)3
1XGE also includes the “Tortle Package”
2Chapters 1 – 5, only
3Taking this option can be done, even if its source
product isn’t your selection for PHB+1
The AL Oracle of War campaign (set in Eberron) has no “+1 Rule”; instead, it permits character options only from the PHB, Eberron: Rising from the Last War, and XGE. Character options from other sources, including the Explorer's Guide to Wildemount book, are not allowed.