We should have the ability to run any code we want on hardware we own

As for the new Android restrictions I assume my Galaxy S20 will be immune to them because it's not getting (major) updates anymore. I'll continue using it as long as I can to avoid this. Does anyone know the most recent Galaxy phone that will be safe from this? I want to get a backup.

It likely won't be safe - they're probably going to enforce it through a Google Play Services update rather than an Android update, which means all previous Android OS versions after 5.0 (Lollipop, released in 2014) will be hit with the changes. In order to bypass that you'd need to install a Custom ROM or stop using and uninstall the Google Play Store entirely (since it's not possible to selectively disable just this).

Android uses Google Play Services updates to update some features or security without relying on manufacturers to update the OS and drivers.

I seriously doubt they can restrict sideloading through Google play services.

But if they do then it’s worse than what I thought.