I don't know. The point is, if you wait for the package manager to update the root hints (whenever that is), is this an advantage over putting in the latest file when you install unbound?
If you install it yourself, you ensure the latest is in place.
Debian packaging is an odd beast. Stretch only gets security updates now and I guess the maintainers feel that outdated root hints are not a security issue.
Raspbian is for the most part Debian, with some additional packages and customization.
Edit: For fun do a search on "Why are Debian packages so old?" and you'll get quite a lot of results.
I just spun up a vanilla debian VM. The root.hints file is from March 2019, so raspbian matches debian.
There appears to be a discrepancy between the official debian git repository and what they are pushing out.
I'm going to (attempt to) contact the maintainer to find out if they only push out changes when the content actually changes, rather than just the timestamp.
I contacted the generic debian package email for dns-root-data but have not gotten a response. I also sent emails to all 3 package managers directly, but still have not gotten a response.
The next time there is an update to the content of the IP or FQDN part of the file, I guess I will have the answer, unless someone knows a better way of contacting the maintainers.
Based on the fact that the file is old, but the content is fresh, I believe that they only update it on a content change. I will continue to monitor the situation for both buster and stretch.