Google has made two changes to Google Search Console in the last 24 hours. Most importantly, the index declaration report now uses mobile first indexing data when available. Meanwhile, Google has issued sunset notifications of old Google Search Console reports when similar reports are found in the new Google Search Console.
Google Index coverage report uses mobile first indexing data. Google has updated the index declaration report to use data as it collects from the first first indexing initiative, instead of using text indexing data for sites that have already switched to mobile first. Google said that this only affects "properties that have been migrated to mobile first indexing."
What has been changed with the index coverage report? Google said that the only data affected in this report is related to the "error chronicles" and "new issues" messages. Looking forward, this index declaration report will reflect the status of mobile first indexing around these issues. Google said that "indexed page count is not affected by this change."
When changed index index report? Starting November 26, 2018, it has been modified for properties that have already been moved to the first first indexing initiative, Google said. You would know if you moved because Google would have sent a message that your site / property has already been moved. If not, Google will still be using indexing data for this report, at least until you receive a notification that your site has been moved.
Google releases some of the old reports. Google has sent messages throughout the old version of Google Search Console, so users know certain reports are missing. Messages can currently be found in the old Google Search Console, Report for Range Report, Rich Performance Report, AMP Report, Country Report, Internal Links Report, Manual Actions Report, Mobile Application Report, Status Report, and Encrypt Error. All of these reports will go away at some point in the future. Google has not given us a date.
The message reads "Important: The report will soon be replaced by the new Search Console. Please update your links to point to the new Search Console." Some of the messages link directly to the replacement report in the new Google Search Console.
Why does it matter? If the index change report changes, if you see changes in errors around November 26, and your site has been moved to first indexing, Google may report these errors. It does not affect your rankings, but it affects the index declaration report. It's only a reporting change and has no impact on your current ranking.
Regarding the notice that Google deletes some of the old Google Search Console reports, Google said that they currently only delete reports that have been reimbursed in the new version of Google Search Console. In some cases, it may mean that you need to learn how to use a new report or change your daily routine and start using the new report of the old one. But overall, Google said that you should be able to get the information you want from the new reports. It only takes time for you to learn how to use them.