Going back and re-reading the statement in the release notes, I’d suggest the wording was ambiguous to the nature of what actually changed:
We now disable mailing list mode via site settings by default so sites don’t experience an unexpectedly large cost from outgoing email.
I would suggest that typically when people think of defaults changing, they don’t think their specific choices would be overridden in the future. As this one went, it’s kind of like when YouTube keeps re-enabling “autoplay next video” on new upgrades of their app after I keep disabling it, or when a marketer keeps sending me email because I only opted out of that one fork of their master subscription list.
This is a good suggestion IMHO to keep user perceptions of Discourse high. Otherwise this change would be seen as something breaking for users, not something to improve the experience of admins.