Google has news on what you will need to do for still being able to sideload apps:
* enable developer option
* confirm prompt that you are not tricked
* restart phone and re-authenticate
* wait one day doing nothing
* confirm with biometrics or device PIN that you know what you are doing
* enjoy the last the few apps that still have developers that are motivated to develop for a user-base willing to put up with this
https://goo.gle/advance-flow