I finally managed to read The Last Language On Earth by @pierskelly cover to cover. Most fascinating read about one intriguing case of human linguistic creativity!

Eskaya is a created script and language of the Philippines. The author succeeds in disentangling the complex historical and ideological context of Eskaya while staying respectful to the views and ideas of its users

https://global.oup.com/academic/product/the-last-language-on-earth-9780197509913?cc=de&lang=en&