I'm not sure about the level of accessibility support in the authoring tool itself; but you certainly can use it to create courses that meet all of Level A, Level AA and some of Level AAA of the WCAG 2.1 guidelines.
All the 'core plugins' (those installed automatically with the authoring tool or framework) have the same high level of accessibility support. See the README for each one for more information.
To what extent 3rd party plugins meet the standards is something you'll need to check with each plugin's owner.
FYI for the best accessibility support you'll want to use either v4.x or 5.x of the Adapt Framework (either by itself or in the authoring tool).