Does it mean that if any app goes off the App Store it won't be automatically deleted from the phone?
What would happen if Casa got removed from the App Store?
You can use Sovereign Recovery, but you can also turn off app deletion on iOS devices. Here's how:
Settings > Screen Time > Content & Privacy Restrictions > iTunes & App Store Purchases > Deleting Apps > Don't Allow
We recommend doing this in advance. Read more about Sovereign Recovery here: https://support.casa.io/knowledge/sovereign-recovery-for-bitcoin-vaults

Discussion
Normally it is not automatically deleted. However, ss soon as you delete it yourself, it wonβt be possible to reinstall it. E.g. Relai, an app still unavailable in my country of residence, which I still have despite its removal from the App Store (long time). I could also transfer it to a new phone (thanks to backup, that is).