Query

https://reshmee.sharepoint.com/_api/v2.1/sites/0358bd78-88da-4d72-b59f-031faa869bc4,2f430014-2d1c-4069-b587-ad55f425c2ab/contentTypes/0x0120D520?$select=isUpdateAvailable

Response

{
    "@odata.context": "https://reshmee.sharepoint.com/_api/v2.1/$metadata#sites('0358bd78-88da-4d72-b59f-031faa869bc4%2C2f430014-2d1c-4069-b587-ad55f425c2ab')/contentTypes(isUpdateAvailable)/$entity",
    "isUpdateAvailable": true
}