I don't think it will automatically uninstall sideloaded apps in your phone, it simply prevent sideloading from unregistered developers. There is an "official" way to circumvent it, doing it directly on the phone is a bit convoluted and require you to wait 24 hours, the other way need a PC and USB cable to install apps with adb--that one is just like usual.

https://android-developers.googleblog.com/2026/03/android-developer-verification.html