I have been an avid user of the KDE Desktop for over 12 years. Some things about the new Plasma desktop are not immediately obvious.
Overall the new Plasma desktop is fantastic. In some cases, the usual tools are missing. KSnapshot, the screen shot utility, is one of those tools. There are a few different projects that I checked out as a replacement, but I landed on using Spectacle - https://github.com/KDE/spectacle. While I think this is getting rolled into Plasma eventually, for now you may have to install it separately. Most distros have a community package available.