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".