Because application security is like an arms race: bad actors are constantly working to find new ways to undo the protections we apply, and we are constantly working to stop them. One of the ways we help protect our customers is by not announcing most changes we make to our protection techniques. That way the bad guys don’t know whether a new release is worth investigating, or if it is, where they should look. This helps ensure that our customers have a longer lead time before the bad guys even find out that they’re blocked.
Historically, our policy for this was just to say nothing – no mention in the changelog at all. (We have years of changelogs with unstated protection enhancements!)