PBN Setup factors, footprints

On other forums there was some discussion about how PBNs are working today. The final decision was that you should setup your pbn more carefully than ever (obviously). But not for passing a manual review but for passing some algorithmic filter that google introduced in the past months.

So, what could be the "footprints of a pbn" that google can filter algorithmically?
In this topic I am not talking about hosting, ip address, logging in, or outgoing links.
Just pure onsite tweaks….

PBN Setup factors, footprints