We'll create fresh WordPress site with Multipart robots.txt editor installed. You have 20 minutes to test the plugin after that site we'll be deleted.
You can edit your robots.txt and add remote content to it.
E.g. you have several sites and want to use a centralized robots.txt.
WordPress handles robots.txt as a virtual URL – just the same way as posts and pages.
So when you browse to https://example.com/robots.txt
WordPress generates robots.txt on the fly.
Development of this plugin goes on on GitHub.