The "Don't show on Reading Pages" checkbox appears when you click the "Edit date" link next to the entry date when editing or creating an entry.

I use it quite frequently, because I make a boring weekly tracking post which is not private but also I suspect not very interesting to most people, so I post it behind a cut. If I'm posting it a day or two late for reasons (this happens more often than I'd like, but eh, done is better than perfect), I backdate it to the right day and then tick that box. I also tag it diligently.