Yes, you could add support for it. To what end? Do you think the usage will be high enough to justify the effort to implement and maintain it?

Even imagining the ideal world where every to-do app implements this, portability of to-do items between apps just isn't that big of a problem. There isn't a big need to move long lists of to-do items between apps.

And in the real world, this won't be implemented perfectly. You'll have the App A implementation, the App B implementation, and so on. Look at Markdown. You'll have headaches migrating your to-do items automatically and in the end, most likely, you'll just end up copy and pasting them. It's not that hard and old to-do items aren't that important to the vast majority of people.