It never "stopped working"... it is just incredibly difficult to do (and always was).

Most nodes are configured to reject double spend transactions with the "txn-mempool-conflict" error. Some nodes (like the ones that the service in your link have setup) will happily accept double spend transactions and attempt to relay them. This doesn't mean other nodes will accept those transactions, or that they will get confirmed.

The issue is whether or not your transaction will make it into the mempool of a miner... unless a miner is willing to accept your double spend transaction then the chances of it being included in a block a practically zero.