Starting with today's release, when a user logs in from a device for the first time, layouts will be created for that device size that are assigned only to that user. And the Repertoire > Layouts list on the website now shows which users each layout is assigned to. So for new accounts moving forward, this should make it pretty easy to remove layouts that are only associated with a particular user.
For existing accounts that already have layouts shared by multiple users, you would have to do some cleanup to get to the behavior of a new account. The Repertoire > Layouts list shows the screen size each layout was created for, and if you hover over that it shows the name of the device it was created from, so in many cases you can use this info to figure out whose device a layout is for and assign the layout only to that user. If multiple users have the same device size, you would have to assign the layout to one of the users and ask the others to click Help > Utilities > Load Standard Layouts to create additional layouts assigned only to them. However, you should check to see if those layouts have been customized first, because then the new layouts would need to have the same customizations applied. At that point it might be easier just to keep the shared layouts. You could at least reduce their user assignments to only the users with that device size.
Layouts can still be shared by multiple users by changing the Users field for a layout, but that is no longer the default.
As a side benefit to this change, users with Added Data editing access will always be able to edit their own layouts, because they will always get a new set of layouts owned by them when they log in for the first time from each of their devices.