Google changed the way their are the Gatekeepers. It now is tied to requiring a software developer ID attached to a real person; Developer Verification. [0]

And how side-loading will have to go through ADB versus just allowing the application to be installed by a file manager.

This is why GrapheneOS and /e/OS have been popping up, along with Linux based alternatives.

[0] https://news.ycombinator.com/item?id=47251763

Google partially walked that back and now says there will still be a way for end users to enable sideloading for apps without developer verification (unclear what that will look like though):

  Based on this feedback and our ongoing conversations with the community, we are building a new advanced flow that allows experienced users to accept the risks of installing software that isn't verified. We are designing this flow specifically to resist coercion, ensuring that users aren't tricked into bypassing these safety checks while under pressure from a scammer. It will also include clear warnings to ensure users fully understand the risks involved, but ultimately, it puts the choice in their hands. We are gathering early feedback on the design of this feature now and will share more details in the coming months.