Alongside Waxed, Handlebar has also been updated for Moodle3.5
https://moodle.org/plugins/theme_handlebar
https://github.com/roelmann/moodle-theme_handlebar
This should again (as with Waxed) be considered to be Beta subject to changes to the Boost theme during Moodle3.5 QA Testing. I will release it as Stable as soon as I can once M3.5 is finalised.