WP Subtitle
WordPress plugin
WP Subtitle plugin

WP Subtitle plugin can be used to add additional information below the post title.

Chap theme has an action that renders subtitles when available. You can use the chap_subtitle filter to specify your subtitle. This hook is used out of the box to look for the subtitle from post meta created by the WP Subtitle plugin, so you don’t need to do any coding and can just install the plugin if you need subtitles.

It works for regular titles, titles in masthead as well as titles on AMP pages. The subtitle is displayed as a Semantic UI subheader.

Subtitles – Add subheaders for posts and pages
WordPress plugin

This theme also supports the Subtitles plugin, how ever it is abandoned and does not support the new block editor (Gutenberg) at this time.