I ran into this warning the other day when I tried adding an animation clip to an object “Animation Component must be marked as legacy”, and there was no obvious way to find and fix the legacy problem… After searching a little bit on the internet, I found this solution to fix the warning:

Select the animation clip in your Project > Go to the Inspector tab and clip on the “hamburger” button > Then select “Debug”.

The inspector options will change into debugging mode and there set the “Animation Type” from 2 to 1. Set the inspector mode back to “Normal” through the hamburger button and it’s fixed! No more legacy warnings!