Can the deep indentation on my neck located under my chin be corrected?

What you are describing is called a cobra deformity. It's most likely that The platysma bands are still present and that the excess needs to be removed and then the two bands sutured together etc. Make sure you see a very experienced revision facelift surgeon for this. 

Thank you for asking. This is a situation that requires in-person exam to determine your needs. Skin, muscle (platysma), soft tissue and also possible salivary glands should be considered to generate the specific right treatment for you. Fat transfer is sometimes needed but should match your anatomy. Hope this helps.