I found one of my previously approved videos still indexed on Pinterest, but its Adobe Stock link now returns a 404 error, which indicates the asset was removed after approval.

Periodically, Stock looks at & purges inventory that no longer meets their acceptance standards. Sounds like you were on the receiving end of such a purge. 

Sorry, but there’s nothing fellow users here can do to help you. As ​@Jill_C said, you’ll find removed assets in your Contributor Portal. Also, you should have received a communication from Adobe Stock about it. Check your email spam folder. 

Look in the “not accepted” tab in your account, where you should find the removed assets.

You are not addressing Adobe here but contributors like yourself and we cannot be of assistance. You should contact contributor support with your concerns via the link at the bottom of your Dashboard.