seaborn uses dashes in lineplot so it would be best if this deprecation could be as soft as possible.

One point I would make is that the line._linestyle attribute does not track all of the information provided about how to draw the dashes:

    def set_linestyle(self, ls):
        if isinstance(ls, str):
            ds, ls = self._split_drawstyle_linestyle(ls)
            if ds is not None:
                self.set_drawstyle(ds)

            if ls in [' ', '', 'none']:
                ls = 'None'

            cbook._check_in_list([*self._lineStyles, *ls_mapper_r], ls=ls)
            if ls not in self._lineStyles:
                ls = ls_mapper_r[ls]
            self._linestyle = ls
        else:
            self._linestyle = '--'

If you're going to be modifying the linestyle code to take the dash patterns directly, it might be good to fix how that works.

@mwaskom thanks for the comment. Which untracked information are you referring to specifically? Note that passing the drawstyle as part of the linestyle has been deprecated in 3.1 (#12442) and is removed for 3.3.

Sorry, I mean that any kind of quantitative dash pattern (passed to linestyle or dashes) gets stored as the simple string "--", with no obvious public access to the actual dash sequence.

f, ax = plt.subplots()
line, = ax.plot([0, 1], linestyle=(0, (4, 1)))
line.get_linestyle()

"--"

See also #17316 and #11797.