People that I’ve asked about this add-on, don’t seem to know about it, so I made this seperate thread to inform.
So, particle hair shape keys panel work the same way as the regular shape key panel, but takes effect on hair particles. For instance, if you want character’s hairstyle to be switched in animation, you can make it happen with this tool. Also, if baking hair physics is too difficult or buggy, you could use this add-on instead.
At least one persion has this add-on in his Blender, and made the video tutorial about it:
Sadly, the download link that’s in description, doesn’t have this add-on. I tried to Google this add-on, but I still didn’t find any download links.
So if anybody has this add-on, could he please make it downloadable, or make another hair shape key add-on that works in all versions?
Short version: this wasn’t an add-on. It was an experimental feature that was ultimately never used. You can read more information about it on the development wiki