This theme is just that: solarized8 with no configuration that only supports solarized terminal colors and truecolor / gui. Since I've completely stripped the 256 color approximation, I've decided to call this fork solarized16.
Solarized8 was perfect, but over the time it shifted from sensible defaults to 256 color approximation and added more options than was necessary.
Flattened could have been nice if it was updated and worked with the plugins that I use.
The original solarized colorscheme code is obnoxiously complicated. Also 256 color approximation is default and there is no support for termguicolors.
In terminal, this theme only uses the first 16 colors. Hence solarized16. You can also think that it's two times more awesome than solarized8 (in reality it is not, the original offers many cool options if you're into that).
Tough luck. Check out solarized8. In my opinion though, 256 color approximation is so horrendously ugly, there is no point including it. You might as well just go for a colorscheme written specifically with 256 colors in mind, like lucius or PaperColors.
I'll probably merge changes from the defaults of solarized8 from time to time.