From cTheDragons:
It appears the username in the settings on the web has not been updated since the changeover. It states:
"We will be transitioning login names to unique, public usernames soon. This username will be used for invitations, @mentions in chat, and messaging."
From shanaqui:

website/common/locales/en/settings.json:
"setUsernameNotificationBody": "We will be transitioning login names to unique,
public usernames soon. This username will be used for invitations, @mentions
in chat, and messaging.",
"changeUsernameDisclaimer": "We will be transitioning login names to unique,
public usernames soon. This username will be used for invitations, @mentions
in chat, and messaging.",
Both strings are in use. I suggest removing one.
~On hold until the staff have chosen new text.~
EDIT 2020-02-26: The staff have chosen new text and it's shown in this comment: https://github.com/HabitRPG/habitica/issues/10967#issuecomment-590968412
The desired fix is to remove setUsernameNotificationBody and use changeUsernameDisclaimer instead everywhere, and to put the new text into it.
Hi. Could I work on this once you have chosen the new text?
Any update on this? If @Chrisyee22 doesn't want to do it anymore, I can do so.
Hi @GiacomoLaw, I'd still like to work on this issue. I was waiting for the new text to be decided upon.
@Chrisyee22 awesome, thanks!
Hi @Alys. Just wondering if the staff has chosen a new text?
@Chrisyee22 No, they haven't yet. It will be posted here when they have.
Is the transition complete? Seems like it is. In which case both should probably be removed, instead of replaced. Unless that area is to be used for explaining what the Username is for and the length/character requirements.
I'll get rid of the text?
@kmacart This issue is still on hold waiting for the staff to comment so
please don't make any changes yet. Once a decision, the "on hold" label
will change to "help wanted".
We do have plans to update the Settings pages this year which will update this messaging. In the mean time, let's update the copy to:
Your username is used for invitations, @mentions in chat, and messaging. It must be 1 to 20 characters, containing only letters a to z, numbers 0 to 9, hyphens, or underscores, and cannot include any inappropriate terms.
I've marked this issue as help wanted.
Hi, could I work on this please?
Sure @jcholewa ! I'll mark it as in progress
Just to clarify - is this making changes only in website/common/locales/en/settings.json and wherever the string is used, or modifying other settings.json files also?
Only the English file, the other languages are handled automatically by our translation service
Great, thanks!
Most helpful comment
@kmacart This issue is still on hold waiting for the staff to comment so
please don't make any changes yet. Once a decision, the "on hold" label
will change to "help wanted".