The goal of this issue is to display the "Row Evolution" icon on all reports, when this report is flattened.
Currently we do not display the Row Evolution icon because the Flatten reports were too slow to generate. Therefore exposing this feature would have been bad user experience in many cases. Now that we have made Flatten much faster in https://github.com/piwik/piwik/issues/6763#issuecomment-77797219 then we could again re-display this icon. I believe that speed was the only reason we didn't display the icon (rather than having to write new code etc.).
The value for users is huge: for example seeing Evolution of metrics for Flattened page urls is priceless.
This bug is causing so much hidden pain so let's aim to fix it in 3.7.0. Row evolution is a key feature and it has to be made available everywhere as it provides huge value in making the most of the data. and it's very confusing to users why the feature is available in hierarchical views, but not flattened. it will be awesome to finally have it!