The current docs do say exec_all can be put in the front matter: https://nbdev.fast.ai/explanations/docs.html#forcing-cells-to-execute
The front matter is different from a “directive,” though. How have you tried putting exec_all in the front matter?
From this, I’m guessing you are aware of the docs linked above and have tried putting it in the front matter. If it’s true that “exec_all” does not appear in the current source code, you may be right that it is not actually implemented. Something similar happened in this case: Remove export-other from directives doc by hugetim · Pull Request #1291 · fastai/nbdev · GitHub
In Sept. 2022, someone else reported ‘exec_all’ wasn’t working, without apparent follow-up: Nbdev v2 & Jupyter Widgets - #8 by tylere