I noticed that the default behavior of Tor browser is to use POST method for all DuckDuckGo searches even when that's not the default DuckDuckGo setting. At first I thought it'd be to protect the search query from the exit node believing that TLS doesn't encrypt the HTTP header but, I was wrong. I found that HTTP Headers are encrypted if the site uses TLS certificates which DDG does and it's equally difficult for exit node to figure the query string no matter which method is used.
So what's the concern that Tor is trying to address by switching to POST?
If it helps, I've captured images of both, GET and POST request headers on DDG.