I recently got the following message from the YouTube API Services team:

Dear YouTube API Developer,

For your use of YouTube API Services, we have sent multiple reminders, which required you to submit a completed Audit Form with (i) fully-functional demo accounts, (ii) screenshots and (iii) documents in relation to your API Client. As a result we have reduced your quota access to YouTube Data API.

You have three (3) days to respond to this message with the requested information, failing which access to YouTube Data API would be affected.

Thanks,

YouTube API Services team

They reduced the quota limit from 25M to 12.5M queries per day:

Everyone is requested to create their own API key and client ID/SECRET values, otherwise youtube-viewer will no longer work.

Check the instructions in README.md for how to create your own API key.

Edit: the default API key of youtube-viewer is now disabled.