The default colour/fill scales like scale_colour_discrete(), scale_fill_continuous() etc. now all produce a scale where palette = NULL and thus theme palettes are invoked. For that reason, we wouldn't need to invoke a special scale here:

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point(aes(colour = class)) +
  theme(
    palette.colour.discrete = pal_brewer("qual")
  )

p

The old way of specifying default scales through options() is reasonably preserved.
Some areas where backwards compatibility does not hold completely:

• Error messages no longer suggest alternatives for the type argument.
• If a scale is used outside plots to map variables, a NULL palette might throw an error.
• The default scale_colour_discrete() scale no longer constructs a scale_colour_hue(), so arguments like h, c and l are not passed on.
  If we're happy with the way the theme() route for palettes work, we could consider deprecating the options() route.

options("ggplot2.discrete.colour" = scale_colour_viridis_d)

p

^{Created on 2024-06-19 with reprex v2.1.0}

Now supports all vanilla non-position aesthetics.
Extension aesthetics can be added by using register_theme_elements(), where fallback palettes can be set as part of the specifications.

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2

register_theme_elements(
  palette.foobar.discrete = function(n) seq(0.1, 1, length.out = n),
  element_tree = list(
    palette.foobar.discrete   = el_def(c("character", "numeric", "integer", "function"))
  )
)

fallback_palette("foobar", discrete = TRUE)
#> function(n) seq(0.1, 1, length.out = n)

^{Created on 2024-06-21 with reprex v2.1.0}

I'll bump this from POC to WIP.

Should now work with the new scales::as_discrete_pal() and scales::as_continuous_pal().
The named version of specifying palettes doesn't currently work until r-lib/scales#448 gets merged.
Bumping this from WIP to ready for review

Yeah I think after we agree on the edition infrastructure we can start marking things with edition_deprecated() or something.