Multiple people lately have asked about this and I think they do not know that an extension exists. I am not the extension creator or maintainer, but I thought that there should at least be a link to this, within the extensions tag, so that people can find it if they do not go to Extiverse.
Mods, feel free to re-tag this post though if the extensions tag should only be used by the extension author.
Here it is. This adds the “Author” label when the topic starter replies. I run this at my Flarum and as of March 2022, it works.