It seems Turtl was already in the pipeline to become added. but dropped out when the forum was changed.
https://f-droid.org/forums/topic/turtl/ <-- 404 does anybody know about the state?
The Sourcecode is GPLv3 https://github.com/turtl/mobile
and it seems they prepare a separate repo https://github.com/turtl/fdroid (
An auto-generated repository for the FDroid project so it doesn’t have to deal with a Cordova build system