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When viewing a user who has their parent user on chat set to Area 51, it still links to discuss.area51.stackexchange.com, instead of the new URL, area51.meta.stackexchange.com. See:
An example that's still true: see Jaydles' chat profile. The chat profile is parented to Area 51, and the link points to discuss.area51.stackexchange.com/users/26/jaydles (which causes browsers to show a security error because there isn't a valid HTTPS certificate for discuss.area51.stackexchange.com).
discuss.area51.stackexchange.com
).