In March 2022 Google deprecated YouTube Mockup Tool. The YouTube Mockup Tool enabled advertisers and agencies to upload videos and see how the ads will look like when they appear on YouTube.
Google still has a mockup tool, but only works with VAST tags. The tool is called YouTube VAST Validator.
Advertisers just need a VAST tag from an adserver (VAST 2.0 or VAST 3.0), and the tool will generate a preview link on YouTube.
According to Google, YouTube VAST Validator was built to verify if a VAST ad tag meet specifications for third party served video ads on YouTube.