As mentioned by @rafaelss95 in #3996 (comment), the PR template right now doesn't ask for an attached issue. It is no surprise then that quite a few PRs today don't have an attached issue.

I propose we switch to a template like this one with a checklist for having an attached issue that was marked as accepting PRs: https://github.com/typescript-eslint/tslint-to-eslint-config/blob/9f32f5a/.github/PULL_REQUEST_TEMPLATE.md

Current template for reference: https://github.com/typescript-eslint/typescript-eslint/blob/a4f95d3/.github/PULL_REQUEST_TEMPLATE/standard.md

(ironically skipping the issue template for this as it is docs, not code)