GET repo doesn't return delete_branch_on_merge key

Using GitHub Enterprise Server 2.20.15 and PowerShell to GET /repos/{owner}/{repo}
But the response doesn’t contain the delete_branch_on_merge key.
The PowerShell command I used:
$repoInfo = Invoke-RestMethod -Uri “” -Headers @{‘Accept’ = ‘application/vnd.github.v3+json’ }
Other keys that are specified in the API documentation are returned, like allow_squash_merge and allow_merge_commit
I have noted that delete_branch_on_merge is not settable until v2.21
But is there a way to get it in v2.20.15?