The default functionality changed within the Feature Update known as Windows 10 version 1709. When the change was first implemented in Insider Preview Build 16215, Microsoft said the following:
Per-App Defaults Settings Page: In the past, when using Settings you
had to start with your file type or protocol if you wanted to make a
change to the default app. That’s changing with this build, and you
can now start with your app, and then see the available options for
what it can handle. To see this new option, go to Settings > Apps >
Default apps > “Set defaults by App”. Choose an app and click ‘Manage’
to see all the file types and protocol associations for which the app
is the default. This page takes the place of the one that had been
available in Control Panel, as part of our ongoing effort to converge
the settings experience.
Update: while we can restore the legacy screen, unfortunately Microsoft has disabled the original capabilities of this Control Panel interface. It is essentially a read-only view of the current default application-to-file type assignments, and it can't actually make any changes.
Update 2: not available on newer Windows 10 Feature Update releases
If you want to utilize this read-only screen, do the following:
Right-click on your desktop and select New > Shortcut
When you are asked to Type the location of the item, paste the following in the adjacent field:
control /name Microsoft.DefaultPrograms /page pageDefaultProgram
You will then be asked to Type a name for this shortcut. The ideal name would probably be
Set Default Programs.
Double-click on your newly created shortcut, and you will be presented with the legacy interface:
After that, you can ensure that the shortcut is readily available by right-clicking it and selecting Pin to Start, or you can copy it to the App List as follows:
Only your App List - %AppData%\Microsoft\Windows\Start Menu\Programs
All users App List - %ProgramData%\Microsoft\Windows\Start Menu\Programs
Source: List of Commands to Open Control Panel Items in Windows 10