What is “Your Google Ads account was cancelled due to no spend” email?

The email notification "Your Google Ads account was cancelled due to no spend" trigger if you have an inactive ad campaign for the past 15 months.
If you’re a Google Adsense publisher and received this kind email from your Gmail account, you don’t need to worry about it. The message is intended for those who previously created an ad campaign in Google Adwords which now Google Ads for the sake of Keyword Planner.  If you did not fund your (test) campaign for the past 15 months, they will consider it as inactive and you will automatically receive notification from Google.
First time reading the whole content of the email notice makes you panic why it happened to your account and the first thing that came into your mind is your Adsense account. Again, don’t worry as it is not related to your Adsense account. Ads will continue to run your websites and YouTube channels.
Your Google Ads account was cancelled due to no spend

Google Ads email notification

“Your Google Ads account 142-XXX-XXX was automatically cancelled on 9/24/18, as it has not spent for over 15 months. Learn more about inactive account cancellation. Ads will stop showing within 24 hours of account cancellation. If you have money left in your cancelled account, we'll refund it to your payment method within 4 weeks. If you’d like to reactivate your account, you can do so by signing into Google Ads account. Learn more about reactivating cancelled accounts.”

If you read the email on your mobile phone, you will notice the new logo of Google Ads (see above image for reference) which now looks the same with AdSense. Again, this kind of email notification is for Google Ads or Adwords and NOT about your Adsense account. You can also read the help links provided on the email or visit Google Product Forums if you want to know more about why it happens.