If they are watching the category, then they got a notification immediately when you posted the topic, before you unlisted it.
If you want it to be in some category that people might be watching, but have it not show up in their notifications, it might work to post it in a private category, unlist it, then move it to that category.
I’m a bit confused here. If I recall correctly, the toggle I shared above was added to address your exact complaint. It allows you to create a topic that is unlisted when it is created, thus preventing notifications. It’s not a workaround…
Whisper and Unlisted are completely different functions. Unlisted applies to topics, where whisper applies to posts. An unlisted topic can only be seen by those with a direct link. A whisper is a post that can only be seen by site staff.