Yup, it's a vestige and should probably be fixed. I thought that Certbot #6755 updated the acme module to just send {} but perhaps it only removed the keyAuthorization and left the remaining fields you're seeing.
That sounds reasonable. Boulder throws away the JSON body right now and acts the same way as what you're proposing.
I think it would be a nice ecosystem bump if we updated Pebble in -strict to outright reject JSON bodies with non-standard fields in challenge POST requests. I'll work on getting an issue filed for that. It already rejects POSTs that include the legacy keyAuthorization but it doesn't go beyond that.