All credit to: https://chiroiu.com/blog/disable-month-year-based-uploads-on-wordpress-multisite/
Disable month- / year-based uploads on wordpress multisite
Wordpress / January 30, 2020
By default wordpress organises uploaded media into month- and year-based folders. Probably a relict from when it was mostly used as a blogging tool. You can disable this easily enough under Settings > Media.
On a WordPress Multisite installation there is no such option. You have to change the setting on the network page itself – so only Super-Admins can do this. As I end up searching for it every freakin’ time here you go:
As Super-Admin go to the multisite’s network into the “Sites” menu and choose edit for the specific site.
Go to “Settings”
Search for “Uploads Use Yearmonth Folders” and set it to 0
Scroll all the way to the bottom and hit “Save changes”