This is prior notice that, per bug 58626,[1] the user ID number will no longer be visible in user preferences.[2] 

User ID is public information, and will continue to be accessible via the API or Special:Redirect. However, since user IDs are only relevant for technical purposes, there's really no reason the average non-technical user needs access via Preferences. 

I'm sure this will get added to Tech News, if it hasn't already. 

1. https://bugzilla.wikimedia.org/show_bug.cgi?id=58626

2. https://gerrit.wikimedia.org/r/#/c/132916/

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