I kind of understand why you want to change the inheritance of subtitle and caption but I'm afraid it would throw users off (especially for subtitle). I now regret we didn't name it "description" or something like that

So should we revert the change in inheritance and manually set the header font to the relevant elements in theme_*() functions?

I'm not sure, but I think it would be best at least for subtitle. I'm fine with caption not inheriting from title, I think that was a wrong choice from the start

After some thought, I think it would be best to have the header_family = NULL be the default.
That way, you don't bake in header_family = "", which is convenient when you're looking to globally change the text font.

Example with the current PR, notice the titles remaining sans serif:

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

p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point(aes(colour = class)) +
  labs(
    title = "Fuel efficiency",
    subtitle = "Described for 234 cars from 1999 and 2008.",
    caption = "Source: U.S. Environmental Protection Agency",
    tag = "A"
  ) +
  theme_gray()

p + theme_gray() + 
  theme(text = element_text(family = "Times New Roman"))

However, this all resolves nicely when the header font is NULL, notice the all serif text:

p + theme_gray(header_family = NULL) +
  theme(text = element_text(family = "Times New Roman"))

^{Created on 2024-05-21 with reprex v2.1.0}

I'm not sure tags should inherit from title... Usually you want these pretty unobtrusive and subdued

It also isn't immediately obvious to me why tags should have title style. I'm happy to swap their inheritance to the root text element if you agree.

yeah, let's change that

Alright, it now looks like so:

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

ggplot(mpg, aes(displ, hwy)) +
  geom_point(aes(colour = class)) +
  labs(
    title = "Fuel efficiency",
    subtitle = "Described for 234 cars from 1999 and 2008.",
    caption = "Source: U.S. Environmental Protection Agency",
    tag = "A"
  ) +
  theme_gray(header_family = "Impact")

^{Created on 2024-07-11 with reprex v2.1.0}