I've just noticed "header.mustache file" in boost theme which comes with the newly released Moodle 3.9 does not exist in template folder anymore.... Although the previous version of LTS 3.5 had this "header.mustache" file in template folder of boost theme. I was customizing this header.mustache file using child theme.
Having said that, my child theme of current boost (on Moodle 3.9) is outputting the code inside "header.mustache". So it is good but I'm just wondering.
Does anyone know why theme "boost" has got rid of that file? Or where the code inside old header.mustache is located now?