You can run this (using https://github.com/ingydotnet/git-hub) to rename all default branches from master to main; run from a fresh, empty repo or the repo you run it from will get all bloated with garbage refs.

ditch-master.sh

#!/bin/sh

set -e

for r in $(git hub repos -r --all); do
  json=$(git hub repo "$r" --json)

  # ignore forks
  if [ -n "$(echo "$json" | jq -r '.["source/full_name"]+""')" ]; then
    echo "skipping $r; fork"
    continue
  fi

  archived=$(git hub repo-get $r archived)
  if [ "$archived" = "true" ]; then
    echo "skipping $r; archived"
    continue
  fi

  # find out if HEAD is master:
  br=$(git hub repo-get $r default_branch)
  if [ "$br" != master ]; then
    echo "skipping $r; default_branch=$br"
    continue
  fi

  echo "fetching $r to rename branch..."
  git fetch -q git@github.com:$r
  git push git@github.com:$r FETCH_HEAD:refs/heads/main
  git hub repo-edit $r default_branch main
  git push git@github.com:$r --delete master
done

updating here, now, so I don't forget and let it drop off the radar again.

ps. pack and unpack in perl are onerous yet awesome.