In the above input,textarea (line-heigh:0.5) /* TEST fonts and custom CSS file */ the typo (line-heigh:0.5) instead of (line-height:0.5) suggests that this is a user-entered custom CSS.
Also that line has a <code class=”svelte-1fz0gqk fade-in”> <code class=”svelte-1fz0gqk fade-out”> which may help in finding the source. To identify the exact plugin, you would need to inspect the for the <code class=”svelte-1fz0gqk fade-in”> in the functions.php file.
Based on the search context, this specific naming convention ”svelte-1fz0gqk” (hash-based filenames) is characteristic of build tools like Vite or Rollup used in the following types of plugins:
Svelte WordPress Plugin Boilerplate: Projects such as dylanblokhuis/svelte-wordpress-plugin or Ebeldev/svelte-wordpress-plugin on GitHub serve as templates where Svelte components are compiled into a single hashed bundle and enqueued via PHP.
Custom Headless or Embedded Plugins: Developers using WPGraphQL to connect a SvelteKit or Svelte frontend to a WordPress backend often generate these hashed assets during the build process to embed Svelte apps into WordPress admin pages or frontend themes.
Your plugin: “Unique Headers: 2.1.3” seems to come close to those in description.
Then again, as this is a legacy site, it may be something left over from the past.
This plugin will help you identify left over bits: “Plugins Garbage Collector (Database Cleanup) ” https://wordpress.org/plugins/plugins-garbage-collector/