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Is there a free offline way to backup app data from apps especially WhatsApp & games which don't have online backup so as to restore them when I reinstall the app on same or another device?
As in migrate to a new ROM? But wouldn't you have to backup your stuff anyway before you flash a new ROM, unless you are dirty flashing.
No , migrate is a app on the play store which creates a zip. file for all your apps and data which you can flash in recovery mode and then choose which apps/data/permissions to restore. Dirty flashing or clean flashing is generally THE recommended way to flash ROMs on many custom ROMs and devices (mtk ones generally REQUIRE it ) and thus clean Flashing is a common scenario to require the use of a offline data backupapp and hence my recommendation .
I am guessing this also needs an unlocked bootloader as I doubt oem recovery comes with such functionality.