I have some thoughts about such feature. Currently I move relevant topics into Announcenments category which is reflected the digest. Users receive news from this category.
I am not sure about the logic which topic is displayed at the top of the digest, I think this might be tuned via some value in the database (probably the most popular topic has some value, somebody could add a comment on this).
Alternatively, in my case, I can force set watching some tags for all users (usually it can be unset by user in
https://example.com/my/preferences/tags). This is not a digest, but users always is notified by admin tags. This can be repeated by some sql script/ rake task. Anyway, this is absolutely unfriendly, but may be justified some corporate rules in small communities.