That’s sort of why I was hoping at least !jupytext --help worked. If it did you could just use the command line, see here. It doesn’t need to show up in the menu for that route. The command line commands will work in the notebook if you put exclamation marks before them.
And if !jupytext --help doesn’t work now, you can just run in the notebook %pip install jupytext and then it will. Or at least it most likely will.
Note the use of the %pip install in the notebook, see here about the use of the newer pip and conda magics being the recommended way to install with pip and conda in the notebook so that you know it installs to same environment the notebook is running.