From what I've read here, here, here and elsewhere, there's no way to catch when my app is being uninstalled. Some of the questions and answers are quite old...
How do I know then if I can remove related content from my mySQL db if it is no longer relevant?
Content created by user with phone number 123456789 is stored online in my mySQL database. If the user uninstalls the app,then the data in mySQL database should be removed as it is no longer of any use and just taking up space and causing confusion.