Well, they were really never in Google Explore (that + you see at the bottom right of the Google tools)- they just went missing when Explore replaced Research. The good news is they are now in Explore. The screenshot below shows you where to find them. After you find a web resource (via Explore), select the dots for the format type and then select “ to insert the footnote.
Here is the notice from Google with some additional information:In September 2016, we launched Explore in Google Docs, Sheets, and Slides to give you the insights, design tools, and research recommendations you need to do your best work. Today, we’re improving upon the feature by allowing you to easily cite those findings. Students writing research reports, analysts crafting whitepapers, and others looking to credit their sources can now insert citations as footnotes with the click of a button in Explore in Docs on the web. You can even change the format of your citation, switching between the MLA, APA, and Chicago styles. For more information on how to use citations in Docs Explore, check out the Help Center.