Being able to install whatever apps you want on Android without any sort of dependency on a Google signature or API was the standard for a decade and a half.

Let's not act like things have always been this bad and thus we should just accept it as the norm, because they haven't, the noose is actively tightening as time goes on.

It is still the standard today and for the foreseeable future. The only difference is that it will also be possible to install an app not distributed through a preinstalled app store on Google Android builds without a warning as long as the APK has a Google signature.