Currently the in-board method for the notifications is hardcoded and
cannot be disabled. This method should be in his own class extending
`phpbb\notification\method\method_interface`.
It also add the possibility, for each method, to be enabled by default (ie:
no entry in the DB => notification enabled).
https://tracker.phpbb.com/browse/PHPBB3-11444https://tracker.phpbb.com/browse/PHPBB3-11967
PHPBB3-11444
Ability to hide notification types from UCP Notification options
(if users do not have permission to use the notification type, or for
whatever reason they should not see it)
PHPBB3-11103