The forum is configured such that users cannot create an account - only attend via invitation.
(apologies, I can’t find the setting to provide the exact title)
The default message seems to be hard coded:
“An account is required. Please ask an existing member for an invite or log in to continue.”
When you update your Discourse instance to latest version, the custom text for login_required.welcome_message will be copied to login_required.welcome_message_invite_only (if it’s not modified).
Since you’ve already added the custom text for login_required.welcome_message_invite_only the above change will not affect your site, so nothing to worry.