Is "don't mix data and power cables" and "don't loop power cables" still valid nowadays?
Yes, its still valid.

Power and data cable coatings are insulation - designed to protect from electric shock and to avoid short circuits.  They do not interfere with magnetic fields.

I do not know what kind of shielding you are talking about - I have never seen any modern power or data cables with shielding that blocks magnetic fields.  Magnetic fields are quite pervasive.

In response to your follow up question:

The size and strength of a magnetic field is relative to the amount of current flowing through the wire within.  For very low voltage wires, like network cables, bundling them together generally doesn't create a strong enough field to disturb the other lines in the bundle.

However, if you loop the cables, you end up running the same current through very nearly the same space - over, and over, and over - and this can amplify the magnetic field that's created, enough that it can interfere with other signals.

With power cables, they run much more current and so create much bigger fields.  Big and strong enough to interfere with the tiny currents going thru the other cables.  Looping them is even stronger and more problematic.