Hi Ömer,
8.11.0 has been released recently.
As Joost wrote, $expand is going to be supported from this version onwards. $expand is mentioned in the improvements section:
“ Published OData services now allow clients to retrieve associated data using $expand
. “
https://docs.mendix.com/releasenotes/studio-pro/8.11
$expand will be supported from 8.11.0 onwards. For earlier versions, it’s not supported.