The Canvas release notes for 2018-08-04 (linked below) say that CSV Export and Import of Custom Canvas columns is now supported so long as the columns are not read only. I have created a custom column with the API, and it is not read only, but it does not allow me to import via Gradebook import. The import just ignores those columns entirely.
I view the Notes column as a custom column too (and the Canvas API seems to agree) but it also is not importable. Are the release notes mistaken?