This option was introduced to fulfill requirement of RFC3920 10.3,
but in practice it was very inconvenient and many admins were
forced to change its value to 's2s' (i.e. to behaviour that
violates the RFC). Also, it seems like in RFC6120 this requirement
no longer presents.
Those admins who used this option to block s2s with their subdomains
can use 's2s_access' option for the same purpose.
Option 'captcha_host' is now deprecated in favor of 'captcha_url'.
However, it's not replaced automatically at startup, i.e. both options
are supported with 'captcha_url' being the preferred one.
The callback is supposed to provide known options and their default
values, as long as the documentation. Passing default values into
get_mod functions is now deprecated: all defaults should be provided
by the Mod:mod_options/1 callback.
Evgeny Khramtsov