With the June 2019 version of Flex you can apply a private status to individual items using either the New form or the Edit form (not the Display form).
Note: This feature can be applied against any item in Flex however it does NOT make private all the child items. So for example, if you make an Opportunity private you will need to make each child Interaction or Task private as well if you wish to privatize a collection of items. This feature is a one by one process.
No one else will see the item which is made private and it will not show in Sub grids or Search results, however the Administrator of your Flex site will be able to see all private items.
To set this up please add the following Setting:
Setting Name: AllowPrivateItems
Setting Value: List of List Items, EACH ON A SEPARATE LINE