Geometry Dash will not get deleted off your device if it is already installed. Geometry Dash has already been removed from the Google Play store and it might get removed off the App Store soon.
However, if you already have Geometry Dash installed on your device, you will be safe and still be able to play it.
So long as you don't delete the game, you will be able to continue playing it.
If you want to play Geometry Dash on other devices that it is not yet installed on, install it now in case Apple removes it from the App Store.
Geometry Dash will be added back to all stores sometime in October 2023, which is the update 2.2 release date.