Hi Juan,
new WS hub_check_registration is added to local_hub on moodle.net as discussed in MDLSITE-5225

This is what you need to do:

1. Create a user on moodle.net
2. Assign this user capability 'local/hub:checkregistrations' (create a new role for it)
3. Add this user to external service "Moodle airnotifier" : https://moodle.net/admin/webservice/service_users.php?id=6
4. Create a token for this user
5. Test by querying something like this:
   https://moodle.net/local/hub/webservice/webservices.php?moodlewsrestformat=json&wsfunction=hub_check_registration&wstoken=YOURTOKEN&url=SITEURL

Let me know if there are any problems.
local_sitecheck is still part of moodle.net but I would like to remove it when we upgrade to Moodle 3.5 (May 2018)