The way we did it in this project is by setting Time.timescale.
We used DoTween in this project so I just used a tween to slow it down. (Smooths the slowdown effect a bit)
I found that slowing down timescale caused stuttering in the motion of the blades, so I also slowed down FixedDeltaTime proportional to the timescale.
The Unity docs describe how to adjust FixedDeltaTime while adjusting timescale
-> https://docs.unity3d.com/2021.3/Documentation/ScriptReference/Time-timeScale.htm...
I hope this helped <3
ps. Adjusting the timescale also slows down the audio so we got a speed ramp sound for free ;)