There is something funny with overriddden macros.
For example, when I went to mailing_list_information via search I get the original macro, not override.
I then override it (adding 'if your lucky...' see below), it shows the original, then an 'administrator change', then the new 'if your lucky...' override.
This 'administrator change' has the customisation I did some time ago "<strong><t>OpenSCAD Discussion Forum.</t></strong>" etc.
The 'administrator change' has the NAML error mentioned in the above link.
Is it because of the error that it does not show as an override?
OK Yes that is the case.
It appears that if one override macro has a NAML error, ALL overrides are disabled.
I removed the offending bit and saved mailing_list_override, then all the others re-appeared.
So the original issue linked above still stands, why did "is_hidden_mailing_list" disappear?