According to Google, your Gmail experience will automatically switch to the new “integrated view” in April. They have not provided us with a specific date for this change, only that it is expected to take place in April.
You should be able to switch back to the original Gmail view from your settings. However, we recommend that you remain in the new view as it will be irreversible later in 2022. For more information on the new view, including the entire proposed timeline and common issues, refer to New Gmail Integrated View Information and Timeline.
To learn more about how to use the new view, visit the Google Help Center.