Automatic cross referencing is more than linkifying ticket ids and commit hashes. Issues, PRs and commits elsewhere that reference an issue/PR show up automatically on its page (think pingbacks in blogging), which apparently doesn’t fit into the mailing list model at all.

Edit: Forgot to mention, even

> It would be nice to write a filter that auto-links tickets and commits depending on the List-Id.

is not possible with sr.ht, due to non-1-1 relationship between mailing list and repository/tracker.