20 dB/Decade - Justification
In a system with a single signal path through it, (a system with no parallel branches, so no lattice filters for instance), also known as minimum phase, a gain slope of -1 or 20dB/decade implies a phase shift of 90 degrees, see the Hilbert Transform.
This means that a closed loop system will have plenty of phase margin if the gain slope crosses 0dB at 20dB/decade. It's common to use rather more than that, up to 30dB total being not unusual in amplifiers and phase locked loops, 45 degrees phase margin will give a little overshoot, but it's fairly tame.
It's worth remembering that in PLLs, the fact that the VCO is controlled in frequency but measured in phase means that even with no loop filter, you have a 90 degree shift and a -20dB/decade gain slope. Adding a single lowpass filtering element will at some frequencies give you another 90 degrees, which with latency and strays become more than 90 degrees. No wonder that the first PLL put together by a noob is almost certain to be unstable (mine included!) and people often surround them with an aura of magic. This has led to my PLL design method which is (a) with no loop filter, set the loop bandwidth via the gain (b) add integrator element(s) as required, breaking away below the loop bandwidth (c) add lowpass elements as required above the loop bandwidth such that (d) the sum of all phase shifts from the loop filter at the loop bandwidth is 45 degrees or less.
A good reason for aiming well below -30dB slope with the elements you can control is that you'll always have some extra phase shift for free that you didn't intend, from stray capacitance, non-zero output impedance, finite gain.bandwidth products etc.
Once you get to -2, you have 180 degrees phase shift, and the system will become unstable. Less extreme than -2, and you have stability.
If you have a system with a -2.3 gain slope and some phase margin left for stability, then you have a non-minimum phase system, or a measurement or description error.