How to restrict GitHub issues to one assignee?

https://stackoverflow.com/q/61781949/14731 asks about assigning more than 1 person per issue. I would like to do the opposite. How do I prevent users from assigning more than 1 person per issue?

Since that does not seem to be natively supported by GitHub, you would need to set up an alternative way, like a GitHub Action.

Said action would checks the number of assignees on an issue whenever it is updated.
If the number of assignees exceeds one, it could automatically remove the additional assignees and/or post a comment explaining the policy.

You can use for instance actions/github-script, which makes it easy to quickly write a script in your workflow that uses the GitHub API and the workflow run context.

For instance, as .github/workflows/restrict_assignees.yml in your repository:

name: Restrict to Single Assignee

on:
  issues:
    types: [assigned]

jobs:
  enforce-single-assignee:
    runs-on: ubuntu-latest
    steps:
    - name: Checkout code
      uses: actions/checkout@v2

    - name: Restrict to Single Assignee
      run: |
        assignees=$(gh api repos/${{ github.repository }}/issues/${{ github.event.issue.number }}/assignees --jq '.[].login' | wc -l)
        if [[ "$assignees" -gt 1 ]]; then
          first_assignee=$(gh api repos/${{ github.repository }}/issues/${{ github.event.issue.number }}/assignees --jq '.[0].login')
          gh api -X DELETE repos/${{ github.repository }}/issues/${{ github.event.issue.number }}/assignees -f assignees="$first_assignee"
          echo "Issue #${{ github.event.issue.number }} is now only assigned to $first_assignee."
        fi        

In theory, you could have a corner case: if two users are assigned at the same time (for example, in the same API request), it will remove both and then re-add the first.
It also doesn't account for the possibility that the first assignee might be unassigned before the action runs.

But as a first draft, you can try it out and see if it mostly addresses your requirement.