I developed this package in 2017-2018 in Unity 5.5.1, and it appears to still work in modern versions of Unity.
You can download this package at the Unity Asset Store (https://assetstore.unity.com/packages/tools/audio/simplespectrum-free-audio-spectrum-generator-webgl-85294), or by cloning this repository.
There's documentation in the SimpleSpectrum_DOCUMENTATION.txt file. I've no idea why I decided to put it all in a flat txt file. Sorry.