I found this
UX: Hide inner site settings sidebar if admin sidebar enabled (#31047)
This experiment hides the list of categories in the inner sidebar for the main site settings page if the admin sidebar is enabled. It also defaults the list of settings to “All” instead of a specific category. Our theory here is that people who use this page are using it to find an exact setting, not to go through the categories one by one. Our admin sidebar also has several groups of important settings already too, so that can be used for browsing. Finally, the input on the page focuses when you load it, so filtering is faster.
Well, that theory is wrong…
I would appreciate it if experiments have to be enabled explicitly…
Luckily the solution is in there, just remove yourself from admin_sidebar_enabled_groups…