Google has news on what you will need to do for still being able to sideload apps:

* enable developer options
* confirm that you are not tricked
* restart phone and re-authenticate
* wait one day
* confirm with biometrics that you know what you are doing
* decide if you only want unrestricted installs for 1 week or forever
* confirm that you accept the risks
* enjoy the few apps that still have developers motivated to develop for a user-base willing to put up with this

https://goo.gle/advance-flow

They even have a video up where they try to make this all sound nice and positive:
@grote i want to stop using inoperative systems like android or ios, that's it
@oatrapado @grote Europeans might come to the rescue in the coming years with creations like FairPhone and Jolla Phone. Maybe if the donations start flooding in, Pine64 might be convinced to put more effort into the PinePhone.
@Frenezul0_o @grote those chinese/european phones are just obsolete and overprized, pinephone another archeological piece at a high price.
@oatrapado @Frenezul0_o @grote at this point, freedom is worth paying for slower hardware