The Moodle app needs to know the value of $CFG->extendedusernamechars in order to validate the username at client side.

The solution applied on MDL-79296 is not enough to fix MOBILE-4400 in the app. As it is implemented, the app will only receive the parameter if it's enabled, but the app also needs to receive it when it's disabled. The reason is that, as it is right now, the app won't be able to differentiate a Moodle 4.4 with the setting disabled from a Moodle 4.3 or older (because in both cases the app won't receive this setting), and we need to be able to differentiate those cases.