A deck you are designing is yours, and until you say otherwise nobody else can open it. What changed is that saying otherwise is now one control on the deck's settings page, with each choice explained beside it rather than left to the word alone.
The three settings
- Private — only you can open this deck. Any link somebody already has stops working.
- Unlisted — anyone with the link can open this deck. The link is the only way in.
- Public — anyone with the link can open this deck, and it will be listed in Explore when that opens.
Those sentences are the control as much as the labels are. Three bare words leave a designer guessing, and guessing wrong here publishes something they believed was theirs.
What somebody else sees
A shared deck has its own address, separate from the one you work at. It shows the deck's name, its charter and its counts, then the cards straight away — no editors, no controls beneath them. Pressing a card turns it over.
You can open that address yourself at any time, and what you see is exactly what a visitor sees. There is no preview mode to trust and no banner explaining the difference, because there is no difference.