The language is remarked in Scripting section of whycardano.com and at https://www.cardanohub.org/en/cardano-vision-paper/:
we have chosen to design a new language called Simon_6
6: Specifics will be released in an upcoming specification. The full language will be supported in the Shelley CSL release planned for Q4 of 2017.
Unfortunately I couldn’t find any information regarding Simon language at https://cardanoroadmap.com/
Have I missed something?