UPDATE:  This video is quite old now, it will still work, but for even better results and the ability to render out with an alpha, you can grab a much more advanced version from my website at:

https://3d-illusions.co.uk/product/eevee-shadow-catcher-with-reflections-material


Regarding the method shown in this video I did forgot to mention how to control the transparency of the shadow.  To do this, on the colour ramp that's connected to the shader to rgb node, change the left colour from black to a gray.

Pretty disappointed with how @unity3d runs on linux these days. I've send ~10 bug reports over the past week alone. Some really stupid UI issues that should be obvious to anybody actually using editor, but also various peformance issues + crashes. I guess I'm spoiled by #b3d

Инстаграм - https://www.instagram.com/anton_lapenko