The initial issue started out as missing KDE packages in the "Select source" drop down of the Fetch Project menu. Since git was not installed KDevelop would not allow the activation of the KDE provider plugin. Same is true for the "Git Support" plugin. This issue was discovered on a fresh OS install with little or no developer packages installed. A column in the plugin menu that specifies unmet dependencies would have been helpful (or at least message either via pop-up or command line). This issue seems painfully obvious NOW that I know the solution.
Changed the title, because not being able to activate the git plugin without git actually makes sense.
I noticed this issue today on Windows 10 while using KDevelop 5.2.3. I had git installed and tried to enable the git plugin. After checking the box and clicking on apply the checkbox would get unchecked again. The issue for me was that git was not in the path. So I agree that some kind of error message that git was not found / not installed would be nice. I could maybe take a look at this sometime when I have time. Since it would be my first bugfix for KDevelop I would probably need some guidance, but I will ask about that later if I start working on this.