We currently don't display repository access change events in anyone's newsfeed. We do display team access changes, so maybe that's what you're seeing. But rest assured, no one can see access changes to your private repo in their newsfeed.
Sorry I wasn't more specific. I was in fact referring to team access changes. And I can confirm that my private account, which I set to follow my work account (noxqsgit), can see team access changes in its newsfeed even though it has no access to any of the teams or repositories.