Many SharePoint users like the old classic experience as much as modern, and use the switching functionality heavily. You can find the switch button in the Site Contents, as shown below:
How does it work?
Microsoft internally is going to save a cookie in your browser called splnu as shown below:
To check the cookies, open the browser developer tool and switch to Application tab. There you will find Cookies category.
Sometimes the link is not appearing at all as shown below:
To bring the default functionality back, open the browser developer tool and run the following command from the Console:
document.cookie = "splnu=0;domain=" + window.location.hostname + ";"; location.href = _spPageContextInfo.webServerRelativeUrl + "/_layouts/15/viewlsts.aspx";
Note: splnu value of 0 will bring you back to the classic experience.
Now you should see your switch back. Happy switching!