Hi motabar,
Thank you for posting in Apple Support Communities.
It sounds like you are referring to the setting found under App Store. See below for details on the App Store settings, including Offload Unused Apps.
Change your App Store settings
Go to Settings
> App Store, then do any of the following:
* Automatically download apps purchased on your other Apple devices: Below Automatic Downloads, turn on App Downloads.
* Automatically update apps: Turn on App Updates.
* Download in-app content in the background: Turn on In-App Content to download content before you first open an app.
* Allow app downloads to use cellular data: Below Cellular Data, turn on Automatic Downloads. To choose whether you want to be asked for permission for downloads over 200 MB or all apps, tap App Downloads.
* Automatically play app preview videos: Turn on Video Autoplay.
* Automatically remove unused apps: Turn on Offload Unused Apps. You can reinstall an app at any time if it’s still available in the App Store.
The above was found in Manage App Store purchases, subscriptions, settings, and restrictions on iPhone - Apple Support.
Reach back out if you still have questions.
Take care!