what has happened to Mission Control in Catalina

What you are referring to is either Dashboard Widgets or Notifications.

Dashboard has been discontinued in Catalina, Notifications is still supported.

Mission Control is something slightly different.

@AuntieJayne - You mention "in Mission Control" as if Mission Control is an app that contains items. Mission Control is a method to view your Mac's desktop. So it never contains anything. Can you clarify your description of your problem? You don't mean the Notification Centre do you? That contains items like you mention, e.g. weather, calculator. My Notification Centre is also working under Catalina as it did under Mojave.

Also, the keyboard shortcut actions defined in Mission Control work for me under Catalina just the same as they did in Mojave. I've not noticed any change in the behaviour of Mission Control for me under Catalina.

It is in system preferences.

Dashboard was discontinued but Mission Control still works for me with Catalina. See: Use Mission Control on your Mac - Apple Support

It is in your Applications folder, if you want it in the Dock just drag the icon for it from the Applications folder

into the left hand side of the Dock.

Yes Dashboard widgets are gone, use the Notification Centre, top right of your screen.