The book of revelation as comfort to persecuted early Christians I have heard* that the book of revelation was written to provide comfort to persecuted Christians in the early days during persecution from Rome. And that the contents are best understood as metaphors for the the end of that persecution, the imagery as evocative of certain persons/institutions of the day.

Is this a commonly held belief, or a doctrine of any churches or movements?

*My grandfather was a minister in the United Church of Christ. He discussed this topic with my mother, when she asked him about it. She then related it to me many years later when I asked her about it. (Personal story included to ward off claims of unsourced opinion.)
Sounds like you are talking about Preterism.

Essentially, this is that all the events described in Revelation have already happened, in the first century. Since Revelation was written towards the end of the first century, if this view is true then its readers would have been reading about events that had already happened.