Implement the "Notifications Feature" in the modern UI as well.
Previously, there was a function to notify the users by email when items has been changed in the classic UI. By the way, I assume that this function is not available in the modern UI at the moment. It is a little inconvenient for us due to we frequently used the function so far. Therefore, I hope the feature of " Notifications" will be implemented in the modern UI as quickly as possible.
Thomas Habersatter commented
The modern UI hides the notification settings. The alerts feature can still be used by adding the following URL part /_layouts/15/MySubs.aspx so the URL would be something like this: https://demotenant-my.sharepoint.com/personal/firstname_lastname_demotenant_onmicrosoft_com/_layouts/15/MySubs.aspx
Then set an alert on the Documents library.