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On PC/Mac save files never get deleted unless you do so manually. It's fine to uninstall the game whenever.

On Android save files are stored automatically as well but there is no guarantee the saves will be kept over longer periods,  mostly because the coming update, the game will be split up into two parts for Android users. (This is because the game is currently pushing the limits of certain Android systems, by splitting the game into two parts everyone wins.)  It's advised to make backups although it can be quite the hassle. Here's how:

Go to Android: SD:/Android/data/com.{gamename}.program/files/saves Alternatively, you can use the file manager to find the correct save map where the Renpy save files belong.

NOTE: On many phones all those files are hidden thus won't show up. If you want to have more control over your phone/files install something like https://play.google.com/store/apps/details?id=pl.solidexplorer2 or any other file explorer, and you can easily find the map where save files belong and make a backup. During a new update, simply reinstall the game and drop the backup save files in said map.