Android - How can I prevent apps from going full screen and hiding the notification bar?

There is an app for your required feature Status bar in full screen

You can use it with any launcher and it is compatible with Android 2.2+

In the app, you can toggle status bar on/off in fullscreen mode.

Alternatively you can use AppSettings xposed module which also has the feature to disable fulscreen mode:

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Or XHalo floating window which creates small floating windows when launching an application instead of full screen.