Check out this backlink

I was doing a competitor backlink and found the following backlink. (on ahref)

http://joachimheck-gws.sandbox.goog...QGCiZOQAg&q=keyword+which+your+post+ranks+for

Looks like you have to change keyword+which+your+post+ranks+for with one of your ranking keywords. (His site was one of the results on the page)

I tried to index this URL through the youtube description and blogger post method. Still, I was unable index. So I'm not sure it works now. Maybe you can give a try. If you were able to index this please let me know.

Also, be careful I have never seen anything like the above site. I have no affiliation to http://joachimheck-gws.sandbox.google.com/.

These type of links are pretty much useless...

Search for inurl:sandbox.google.com

This is so weird
Don't think this type of links that simply link from the own SERPs engine provide great value if anything but sometimes the engine has major failures, who knows

Check this: ggp.sandbox.google.com
Links to Stadia

Maybe Joachim Heck URL in Google Web Server a Google dev with his own sandbows url for testing purpose