Latest Google Algorithm: Mobile Interstitial & Popup Change
Mobile Interstitial & Popup Change: Pages with intrusive popups or interstitials, that cause poor user experiences, are now downranked in the Google mobile search results.
Google is only applying this change to mobile searches only. So you can still use interstitials on desktop without issue unless you have a desktop-only site.
According to Google's official announcement, the following examples would be flagged:
A mobile popup that covers the main content, either immediately or after a period of viewing the page
Displaying a standalone mobile popup that the user has to dismiss before accessing the page
A welcome mat mobile popup where the above the fold portion of the page pushes the original content underneath the fold.
This only applies to pages accessed directly from search results
Google is looking for a mobile interstitial or pop up that is being triggered after the searcher goes from Google search results to that specific page. Google is not looking at pop ups or interstitials displayed after the initial page viewed.
If a website visitor clicks through from a search result and then decides to navigate elsewhere on your site, you are free to engage them with a pop up or interstitial.