Sorry, this is absolutely incorrect. IEEE802.3 (the Ethernet spec) has very specific requirements for isolation as well as resistance to current and voltage spikes. This is all to maintain performance in variable EMI and RFI scenarios as well as for electrical safety. In order for something to be called Ethernet it has to follow these specs. A while back one of the revisions to the Rasberry Pi left out the transformers by accident and the manufacturer had to recall all of them.
Typical transformers are like these (not an endorsement, just the first thing that popped up in Google):
https://eu.mouser.com/new/halo-electronics/halo-ethernet-transformers/
The packages are small and handle anywhere from one PHY to 12 or 24 depending on the specific design. Open up any product with Ethernet and you’ll find some variant of these. If there’s no transformer it’s not following the spec and is also a potential electrical safety hazard.
The reason I recommend against shielded cables is that they do connect the ground plane of two devices. In the real world (the billions of Ethernet implementations which don’t involve audiophiles) this isn’t an issue as shielded cables are only used in very specific situations (typically within a rack) so the issue of differing electrical potentials is minimized.