github_open_branches.rb

always_preserve_branches = [
  'main',
  'staging',
  'development',
  'ci/data-mover',
  'ci/data-mover-test',
]

a,b,c = Open3.capture3("git branch -r | grep -v '\\->' | cut -d'/' -f2-")

branches = a.split("\n"); branches.count

lim = branches.size  # x=10000
branches_states = branches[0..lim].map do |branch|
  a,b,c = Open3.capture3("gh pr list --head \"#{branch}\" --state all ") ;
  a
end
#branches_states.size

# preserve PRs that are Open, Draft, and keep branch name with them.
open_and_draft = branches_states.map
  .with_index do |l,idx|
    l.split("\n").select do |l|
      l.match(/OPEN/) || l.match(/DRAFT/)
    end << branches[idx]
  end
  .select{|ll| ll.size > 1 }

pr_and_branch = open_and_draft.map do |ll|
  [
    ll[0].split("\t")[0], # get PR number
    ll[1] # repserved branch name
  ]
end

apbx = (
  [nil] * always_preserve_branches.size
).zip(always_preserve_branches)

open_branches_sorted = (
  pr_and_branch + apbx
).sort { |a,b| b[0].to_i <=> a[0].to_i }

# code to iteratively remote-delete all branches that are not in the list of open_branches_sorted

puts open_branches_sorted

with the remote-delete operation, this would delete all remote branches that were not open as PRs, or in the safe list.

In running this, the operator needs to be aware of CI and other integrated services, which may create-and-remove branches programmatically in the repo, as these operations could then fail.