This fails because the 'linestyle' entry was never made in the info dictionary. Also, I would rather not add things to this dictionary. It was only meant to be a stopgap solution while the axes class was getting normalized with regular axes. It would be nice to implement grid line styles, but not like this. I would rather it be done through the normal getters/setters (whatever they would be).

Yes, the original grid field does not have the linestyle sub field.

        self._axinfo.update({'label' : {'space_factor': 1.6,
                                        'va': 'center',
                                        'ha': 'center'},
                             'tick' : {'inward_factor': 0.2,
                                       'outward_factor': 0.1},
                             'ticklabel': {'space_factor': 0.7},
                             'axisline': {'linewidth': 0.75,
                                          'color': (0, 0, 0, 1)},
                             'grid' : {'color': (0.9, 0.9, 0.9, 1),
                                       'linewidth': 1.0},
                            })

But the line I insert surely works :-).
To keep consistency, I also add a default linestyle in self._axinfo.update

As far as I know from the comments in the codes, the whole _axinfo is a stopgap solution.
so my suggestion is to accept my PR to enable user customization in a dirty style just like the color/linewidth fields.
And when the whole _axinfo related codes is refactored by the main maintainer, the linestyle field related codes are also refactored together with the existing color/linewidth field

I think I am with @WeatherGod, user code should not be touching _axinfo as we can change that with zero warning as it is not part of the public API.

@chenhouwu Can you also update the grid method in Axes3D to pass the kwargs through to the Axis objects?

Replaced by #5585