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Reusable Checklists could be done with a simple text document that you duplicate every time you need it no ?

For sure. Though many tools don't read .txt (or .md/.markdown) files.

GitHub and GitLab support (multiple) Issue and Pull Request templates:

Default: /.github/ISSUE_TEMPLATE.md || Configure in web interface

/.github/ISSUE_TEMPLATE/Name.md || /.gitlab/issue_templates/Name.md

Default: /.github/PULL_REQUEST_TEMPLATE.md || Configure in web interface

/.github/PULL_REQUEST_TEMPLATE/Name.md || /.gitlab/merge_request_templates/Name.md

There are template templates in awesome-github-templates [1] and checklist template templates in github-issue-templates [2].