Have they actually solved the (non-noise) environmental concerns of SST? From a NYT article a few years ago:

"According to a 1999 report by the Intergovernmental Panel on Climate Change, introducing 1,000 commercial SSTs more advanced than the Concorde could make the climate impact of airlines balloon by 40 percent by midcentury. That’s on top of the tripling of emissions already expected as more travelers take to the sky."

https://www.nytimes.com/2018/06/11/opinion/reviving-superson...