Using _extends with .github repos

Bug Report

(Opening a new issue as instructed on https://github.com/probot/probot/issues/1294.)

Current Behavior I was hoping that it would be possible to use a .github/in-solidarity.yml file inside an organization-wide .github repository, to have bot settings consistently applied across an org. That org-wide config file would contain _extends: ietf/terminology, to pick up the configuration from https://github.com/ietf/terminology/blob/main/.github/in-solidarity.yml.

(I am trying to do this with in-solidarity-bot – also see https://github.com/jpoehnelt/in-solidarity-bot/issues/416 – but am told this is an issue with probot.)

That seems to fail, however, with an error that I don't know what to do about:

configuration is invalid: [
{
"instancePath": "",
"schemaPath": "#/additionalProperties",
"keyword": "additionalProperties",
"params": {
"additionalProperty": "_extends"
},
"message": "must NOT have additional properties"
}
]

Expected behavior/code I would have expected the above setup to work.

Environment I don't know what probot version in-solidarity-bot uses.