Location settings work in a somewhat non-intuitive way. As long as an app has not yet requested your permission, it will not be listed in your device settings and it can not be disabled. Settings only allow to change your preferences later.

The first time an app tries to access your location, it must ask for your permission. Apps won't use your location until they have asked for your permission and you grant permission.

In case you have not used this feature, you would need to first open an article and create an annotation. This will show the corresponding location permission dialog: tap on "Don't allow" and it should be disabled in settings.

Likewise, if you have already granted the location permission and changed your mind, you can update your preferences from Settings > Privacy > Location Services. Scroll down to find the app, tap the app and select an option.

For more details, see Apple Support:
Turn Location Services on or off
About privacy and Location Services

You can uninstall Articles + like any other app on your iPhone, iPad or Mac.

On your iPhone or iPad, touch and hold an app from the home screen until all apps start to jiggle. Tap the remove icon in the upper-left corner of the app and confirm deletion.

If you have enabled the Cloud Library, data will still be stored in your personal iCloud account. In case you want to completely remove everything, you will need to manually delete it from the device settings.

Please note that this action can not be undone and will delete your articles from all synced devices:
Device Settings > Apple ID > iCloud > Manage Storage > Articles: Delete Data

On your Mac, move the Articles.app from the Applications folder to the trash. Local data will still be stored in the app's container.

Open Finder or the Terminal to manually delete it:
/Users/Your-User/Library/Containers/Articles