Milkdrop presets were, in almost all cases, not released under any specific license. Theoretically, each preset author holds the full copyright on any released presets. Since the presets were freely released and have been used in so many packages and applications in the past two decades, it is safe to assume them to be in the public domain.
If any preset author doesn't want their own creation in this repository, please contact the projectM team and we will remove the preset(s) from future releases.