Google’s Mobile Interstitial ranking demotion went live on Jan 10th as confirmed by Google’s John Muller in a tweet.

Vital details you must be aware of:

Demotion has been applied on a page by page basis.
Giving legal notices using popups is now fine.
However, delayed popups will come under the radar.
Language popups for the global website will get affected from it.
Keep in mind that it will lead to a lower ranking, the sites will not get penalized.
This demotion will get triggered by the on-scroll popups too!
It has nothing to do with Mobile friendly test.
Web sites will not get negative rankings for showing exit popups.
Action bars, slide-ins, and banners are allowed.