I think lots of people have been where you are in terms of stopping wedding/engagemet plans. More common than you think. And for varying reasons. Life happens!
I'd say something like 'oh it has been postponed' or 'we are changing the date'. They don't need to know that your plan may be indefinite. I'm all about just stopping the topic when folks ask things I don't want to discuss...and moving along in a graceful way.
No one has to no the full story but those involved.