I did some testing per @harvos
I've configured my Devices to use a "rec709 only" state.
Shield is set to "rec709"
Dynamic Range on the Shield is set to "Limited"
Tested both 4K and 1080 resolutions
Sony Display is set to "rec709"
RGB on my Sony Display is set to "Limited"
Color Depth on Oppo 203 set to "8-bit"
Color Space on my Oppo 203 is set to "RGB Video Level"
HDR on Oppo 203 is set to "off(bt.709)
Output Resolution for Oppo 203 is set to "Source Direct"
Via Kodi, when playing 4K/HDR Content, HDR is of course not displayed and it is showing in SDR with proper tone mapping.
Reading the information from my Oppo 203's displays rec709 SDR
Netflix and Amazon only display 4K and not HDR for titles with HDR as a result of forcing the Colorspace to rec709 as expected.
((((Removing the Oppo 203 from the mix, the Shield is able to display the same content with the same tone mapping.))))
(((If I set the Resolution for the Oppo 203 to "Auto", it will produce a washed out image, Black Bars look gray, for example.)))
Hope this helps.