Move the chat to the hidden list and set a PIN. The main list no longer shows that row. The decoy PIN opens an empty hidden list if someone demands to look.
A chat PIN is lighter: the row can remain, and the messages stay hidden until the PIN is entered. Blur the preview when you only need the last line covered.
Use it with a timer
Hidden does not by itself delete history. Add disappearing messages when you also want the thread to shrink. View-once is for a photo that should not remain in the thread after it is opened.
On a borrowed computer, sign out when you leave. The account page lists devices and can sign one of them out, or sign out every other device.