This changes the introduction as follows:

Removes the generic definition of web accessibility because it does not explain the page’s current status.
Removes the dated W3C quotation because the current accessibility guide should introduce accessibility concepts.
States clearly that the guide is archived.
Warns that individual pages may contain obsolete technical guidance.
Directs current development to the supported documentation without pretending that every replacement page already exists.
Retains a route for organizations that must temporarily maintain Drupal 7.