They're bash/shell- and bin-dependent commands rather than Git commands. I use Nushell.
Transformed to Nushell commands:

• The 20 most-changed files in the last year:
  git log --format=format: --name-only --since="1 year ago" | lines | str trim | where (is-not-empty) | uniq --count | sort-by count --reverse | take 20
• Who Built This:
  git shortlog -sn --no-merges
git shortlog -sn --no-merges --since="6 months ago"
• Where Do Bugs Cluster:
  git log -i -E --grep="fix|bug|broken" --name-only --format='' | lines | str trim | where (is-not-empty) | uniq --count | sort-by count --reverse | take 20
• Is This Project Accelerating or Dying:
  git log --format='%ad' --date=format:'%Y-%m' | lines | str trim | where (is-not-empty) | uniq --count
• How Often Is the Team Firefighting:
  git log --oneline --since="1 year ago" | find --ignore-case --regex 'revert|hotfix|emergency|rollback'

/edit: Looks like the lines have whitespace or sth. Replaced lines --skip-empty with lines | str trim | where (is-not-empty).

def "gits most-changed-files" [] { git log --format=format: --name-only --since="1 year ago" | lines | str trim | where (is-not-empty) | uniq --count | sort-by count --reverse | take 20 }
def "gits who" [] { git shortlog -sn --no-merges }
def "gits who6m" [] { git shortlog -sn --no-merges --since="6 months ago" }
def "gits fixes" [] { git log -i -E --grep="fix|bug|broken" --name-only --format='' | lines | str trim | where (is-not-empty) | uniq --count | sort-by count --reverse | take 20 }
def "gits aliveness" [] { git log --format='%ad' --date=format:'%Y-%m' | lines | str trim | where (is-not-empty) | uniq --count }
def "gits firefighting" [] { git log --oneline --since="1 year ago" | find --ignore-case --regex 'revert|hotfix|emergency|rollback' }