Organizing committee of conference announced publication of papers, but one year later, this hasn't happened. What to do?
An email to the conference or program chair would certainly be appropriate. You don't need any format for it. Just say you are wondering if the book is still in progress and when it might be expected.
As Solar Mike implies in a comment, the coronavirus might have disrupted this, like many other things. But it can also take a while to produce a physical book, especially for conference proceedings when not done by the association itself. The publisher has higher priorities, most likely.
I know of one conference where the papers from one annual meeting are only available at the next. The publisher is a university press.