OwlGreen Flarum's discussion titles are just plain text, they aren't connected to the TextFormatter API.
The most straightforward implementation for such a feature might be to store a flag on the discussions
table, make it editable through a new UI in the discussion composer, expose it through the API and extend the DiscussionListItem
javascript component to render a label with the value.
For some use cases, you might be able to re-use secondary tags or terms from the Taxonomies extension as the storage system, or even as a full solution with just some custom CSS to move the tag or term around in the discussion entry.
Implementing actual formatting via TextFormatter or other means in the title itself would require a lot of work, and a lot of backward compatibility for every text-only part of Flarum like notification summaries and emails.