To compact my oplog and reclaim 300GB of disk space right after I finisihed migrating a database to Atlas using mongomirror, I had to open a support case. With that we were able to reduce our cluster tier thanks to lowering the disk tier (Azure deployment)

It would be nice to do it myself to be faster without opening a case with severity 4 that will take me at least 2 days to accomplish that. Also, I assume that people without a subscription can't do it since you have to open a case which is exclusive only if you have an Atlas Pro subscriptions.