Consider re-adding link color support for classic themes

[carolinan]

What problem does this address?

Originally reported here: https://core.trac.wordpress.org/ticket/55028

Before 5.9, classic themes could support the link color option with the help of add_theme_support( 'experimental-link-color' );
From 5.9, I believe there is no way for classic themes to have link color option without adding a theme.json file?

In WordPress 5.8, this setting did work without Gutenberg active:
https://core.trac.wordpress.org/browser/branches/5.8/src/wp-includes/class-wp-theme-json.php#L1226

In 5.9, it is removed: https://core.trac.wordpress.org/browser/branches/5.9/src/wp-includes/class-wp-theme-json.php

In the Gutenberg 5.9 compat file, we can find a related comment:
https://github.com/WordPress/gutenberg/blob/trunk/lib/compat/wordpress-5.9/class-wp-theme-json-5-9.php#L1923

What is your proposed solution?

Re-add theme support, but no longer as "experimental".

[desrosj]

I'd like to see this solved in the broader context of "moving features and functionality out of experimental" and "not merging experimental features into Core" (#40316). Is there any reason why the block editor cannot implement something to keep track of things like this, fire __doing_it_wrong() errors when the old way is being used to inform developers that their code needs updating, and then attempt to seamlessly use the non-experimental way?

I don't think that question can be answered in the context of just this report, but wanted to mention it.

[carolinan]

I agree that the problem was that

1. the theme included experimental feature in the first place
2. in 5.8, the feature was available in core, but still named experimental
3. in 5.9 it was removed -without replacement or doing it wrong notice.

[carolinan]

I have hidden my comments on this ticket that mentioned that the link color can be re-added to the classic themes using theme support, because the theme support only works if Gutenberg is active, and has side effects like enabling block gap.
The theme support was tested in core but was eventually reverted because it enabled block gap, which does not work in classic themes.

I suggest enabling the link color by default for all theme types.
Developers need to be able to disable the link color option in case they have clients that should not be able to edit it.