The Wicker man

 A Roman myth

Since Roman times, the story has been they put the Druids to the sword because they practised human sacrifice; including burning people in wicker effigies.

At best, this was propaganda as an excuse for their slaighter; more likely it was lies to subjegate any possibility of an uprising.

Why do I think it never happened?

The science of the project makes it impaisauable. I am not saying the crafts needed were not available, they were, sadly they are no longer practiced.

Let us view the physics of the topic.

People to this day burst into flame for no apparent reason, but the heat needed to turn flesh and bone to ashes is so intense a naked flame won't do.

What would the accelerant be?

You need a substance that retains heat long enough for the flame to ignite the willow. 

Then we have the willow itself, to bend it into shape the wood has to be kept moist, or it will break. Moist wood will not burst into flames, it will smoulder.

Location,location, location

To set the figurine up, you need a large open area, which makes the wind a factor; even in seciluded places there are high winds. If you beat the winds, you have to contend with rains.

If they were victims of this practice they probably died from suffication, rather than being burned alive.

You need to recall we are talking about people who put Christians against animals for sport.