Those who haven't come to the Mac world so recently should remember Dashboard, an environment part of the operating system where users could install and position numerous widgets with functions of various calculator, converter, weather forecast, Post-it notes, mini-calendar and so on.
The Dashboard first appeared on Mac OS X 10.4 Tiger in 2005, and even when invoked appeared on a layer above all on the screen. In Mac OS X Lion 10.7, in 2011, it came to work “laterally” as a Space part. Already on OS X Yosemite 10.10 in 2014, it was disabled by default in the operating system but still existed on it.
From this year, with the macOS Catalina 10.15Dashboard is dying all the time because, I imagine, much of it was still 32-bit written, and the new macOS would be totally 64-bit.
I know people who still use and like Dashboard (* cof * Edu * cof), but his death was more than natural and expected. All the widgets that still exist were created and developed years ago, and Apple even abandoned the Dashboard widget store long ago. Time to move on.
For those who have a lot of Annotations registered in Dashboard, there's nothing to worry about: they will be automatically imported into the MacOS Annotations app (which is also a good old note).
The closest replacement that exists today in macOS to Dashboard is its own side Notification Center, which has widgets on the Today tab. The concept is quite different, but in practice several things work similarly.