Ars Technica has your answer:For those of us who actually use the Dashboard and have STUFF on there, like sticky notes with actual information on them - what happens after updating? I know the Dashboard is gone, but what happens to the active widgets?
"The bulk of Dashboard's widgets (including the weather, calendar, world clock, and calculator) could already be replicated in the Today view in the Notification Center. If you had sticky notes stored in Dashboard, upgrading to Mojave moves them to the Stickies app. I suspect a small faction of Dashboard die-hards will lament the demise of a treasured workflow or declare these substitutes to be Not Good Enough, but the rest of us won't notice."

macOS 10.15 Catalina: The Ars Technica review
iPad apps come to the Mac and old 32-bit apps go away in a wide-ranging update.
