Github’s web interface does not offer an option to bulk delete deployments and action runs. But it’s possible to do so through the CLI tool.

Clean up deployments

user=<user> repo=<repo>; gh api repos/$user/$repo/deployments --paginate -q '.[].id' | xargs -n1 -I % gh api --silent repos/$user/$repo/deployments/% -X DELETE

Clean up action runs

user= repo=; gh api repos/$user/$repo/actions/runs --paginate -q '.workflow_runs[] | select(.head_branch != "master") | "\(.id)"' | xargs -n1 -I % gh api --silent repos/$user/$repo/actions/runs/% -X DELETE