Per these two issues, the anchor button is currently available in the Editor, but does not function because the D8 Editor Advanced link does not have a Drupal 7 port.
Users will be confused why it is an option but does not function correctly.
https://www.drupal.org/node/2743091 →
https://www.drupal.org/node/2743547 →
Postponed
1.0
CKEditor integration
Not all content is available!
It's likely this issue predates Contrib.social: some issue and comment data are missing.
The broken anchor button has been removed and now matches Drupal core. Marking as postponed on the core issue to add anchor support.
It's possible to use the Editor API or a contrib module, such as the one linked in the original post, to add support for anchors in the meantime.