How “official” is Explorer’s Guide to Wildemount?

What’s the state of Explorer’s Guide to Wildemount? Is it first-party or third-party? Was it playtested? Is it an AL-legal sourcebook, can I use it for the "+1" rule?

Related:

  • Is Matt Mercer's homebrew setting of Exandria now considered an official setting, as of the release of the "Explorer's Guide to Wildemount"?
  • How "Official" is the Blood Hunter class?