How to plot Nyquist Plots?
A system with smooth rolloff and numerous poles will become a spiral, if plotted on a polar plot.

If you have only 90 degree phase shift, then the low-freq gain, assumed to be huge, starts far out from the center point (the gain-is-zero point), rotates over to the 90 degree region and plunges down to ZERO. There is no stability issue here.

The classic purpose of Nyquist is to examine how this polar plot behaves near the [Mag=1/Phase=-180 degree] point. For stability, perhaps marginal and ringy but stable, the plot needs to be inside the magnitude=1 point at 180 degrees.