My System Preference window moved and isn't in the center anymore

In macOS Catalina you can use the following command in Terminal after closing System Preferences.app to delete the currently stored window position:

defaults delete com.apple.systempreferences "NSWindow Frame Main Window Frame SystemPreferencesApp 8.0"

This will delete the key including the window position.

Afterwards relaunch System Preferences.app and the key will be restored with a default position. The window will be centered in the East-West direction and slightly moved to the North. This is intended by Apple and indicates visual prominence.

In case the key name changes after a system update (e.g. to "NSWindow Frame Main Window Frame SystemPreferencesApp 8.1") you will get an error after entering the above command:

Domain (com.apple.systempreferences) not found.
Defaults have not been changed.

Then you have to search for the new proper key name by issueing:

defaults read com.apple.systempreferences

which will result in a long list:

...
NSQuitAlwaysKeepsWindows = 0;
"NSWindow Frame DestinationSheet" = "643 525 416 311 0 0 1680 1027 ";
"NSWindow Frame Identity Picker" = "673 573 357 263 0 0 1680 1027 ";
"NSWindow Frame Main Window Frame SystemPreferencesApp 8.1" = "9 476 668 548 0 0 1680 1027 ";
"NSWindow Frame NSNavPanelAutosaveName" = "463 333 799 448 0 0 1680 1027 ";
SecurityPrefTab = General;
...

The relevant key is the "NSWindow Frame Main Window Frame SystemPreferencesApp ..." key.

The "updated" command would then be:

defaults delete com.apple.systempreferences "NSWindow Frame Main Window Frame SystemPreferencesApp 8.1"