Ginny Marvin at Search Engine Land covered Google testing AMP labels on search ads, AdWords ad listings. She shared some screen shots of the AMP lightening bolt on the search ad in the mobile search results.
Here is another screen shot from Frederick Vallaeys on Twitter:

Google has rolled out AMP landing pages for search ads last September while beta testing it months before hand.
The AMP team posted this on Twitter as well:

Interesting 😃https://t.co/y6c6onxc73
— AMP Project (@AMPhtml) June 13, 2018
Forum discussion at Twitter.

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