Mea culpa. While fixing T64520 almost a year ago (!) I added a reference to this message in SocialProfile/UserProfile/SpecialUpdateProfile.php but forgot to write an actual message string and add it to SocialProfile/UserProfile/i18n/en.json. Oops!

This message is shown only when Echo is installed (=the EchoEvent class exists) and it's meant to point users to Special:Preferences#mw-prefsection-echo as that is the master special page for controlling what notifications are sent out and when. Any thoughts on the specific wording of the message? (Patches welcome!)

How about something like:

Email notifications are handled by the Echo extension. You can change these in your [[Special:Preferences#mw-prefsection-echo|preferences]].

Change 392692 had a related patch set uploaded (by Jack Phoenix; owner: Jack Phoenix):
[mediawiki/extensions/SocialProfile@master] Add missing i18n message "user-profile-preferences-emails-manage"

https://gerrit.wikimedia.org/r/392692

Change 392692 merged by jenkins-bot:
[mediawiki/extensions/SocialProfile@master] Add missing i18n message "user-profile-preferences-emails-manage"

https://gerrit.wikimedia.org/r/392692