The Boolean mergeable attribute will remain to indicate whether the pull request can be automatically merged.
The value of the mergeable attribute can be true, false, or null. If the value is null, this means that the mergeability hasn't been computed yet, and a background job was started to compute it. Give the job a few moments to complete, and then submit the request again. When the job is complete, the response will include a non-null value for the mergeable attribute.