D16373 changes the logo layout and will break installs which have set ui.custom-header and require them to reconfigure the option.

Configuring this option is currently unreasonably difficult. It should be a simple "pick a file" sort of thing, but requires manually executing database commands (see T4214 and T4214#98401, and I think you may also need to go explicitly set the View Policy to "Public"). It may be a bit less bad than this after sorting out the CDN stuff in T8588, but even needing to go look up a file ID is pretty junk.

ui.custom-header also currently references T4214 explicitly in its configuration description, but that task is unlikely to be of much use to anyone.