(For reference, this edit changed GFDL to CC BY-SA on Project:Copyrights.)

https://meta.wikimedia.org/wiki/Licensing_update provides additional context.

Does the GNU Free Documentation License (GFDL) apply to content on mediawiki.org (outside the Help namespace)?

[Disclaimer: IANAL] I'd say that all content, except for the Help: namespace content under Public Domain, is always at least under CC-BY-SA 4.0 (since lately). Some content might also in addition be still dual-licensed under GFDL unversioned (as GFDL versions before version 1.3 were not compatible with CC-BY-SA if I remember correctly).

Terms of Use section 7g describe the implications quite well:

If the text content was imported from another source, it is possible that the content is licensed under a compatible CC BY-SA license but not GFDL (as described in "Importing text," above). In that case, you agree to comply with the compatible CC BY-SA license and do not have the option to relicense it under GFDL.

Thanks for the clarifications! I've updated the task description with to-dos so that all pages that reference either GFDL or CC BY-SA reference both licenses.

Since the site footer is global, I've removed it from the scope of this task. I assume the current footer text was chosen for brevity, with GFDL covered in the link to "additional terms".