Ahoy CommRel -- my accomplices and I are planning to upgrade the [[ https://en.wikipedia.org/wiki/International_Components_for_Unicode | ICU ]] library on MediaWiki app servers from version 57 to version 63. This is a prerequisite to upgrading the OS and other software on those machines, and allows developers to use new internationalization features including support for Unicode 11.
There will be one user-visible effect: Categories listed at the bottom of articles will be displayed out of order during the transition. This will last up to a few hours on smaller wikis, and longer for bigger ones, up to the neighborhood of a week for enwiki. (The reason: ICU functionality is involved in multi-lingual sorting keys, among other things, and those sorting keys will change with the new version. Pages whose categories were sorted with the old ICU, but are displayed with the new one, will be shown out of order until they're updated. After bumping the ICU version we'll run a maintenance script to update each page; last time, it took about a week to get all the way through enwiki.)
We don't yet know our exact timing for this. We expect to start the upgrade no earlier than Monday Nov 16, and finish fixing sorting keys no later than Friday Nov 27. We'll update you here as soon as we have something more precise.
For your reference: More technical details in the parent task T264991. CommRel task from last time is T189486.