AFAIK, data is not removed from a DataNode when you decommission it. Further writes on that DataNode will not be possible though.
When you decommission a DataNode, the replicas held by that DataNode are marked as "decommissioned" replicas which are still eligible for read-access.
Generally speaking, decommissioning the dataNode stops the NameNode from putting new blocks on the DataNode being decommissioned while redistributing them so you are never under-replicated.
After the decommissioning is done all copies of blocks on the decommissioned data node would have been replicated to the other nodes, remember the data is not deleted but replicated so you will need to format the Filesystem to completely clean the decommissioned data node.
If we decommissioned a datanode, then namenode will look for the blocks from that decommissioned datanode. If Namenode not receiving any update from that datanode, then namenode mark that datanode as dead datanode after 10m. 30sec and replicate the under replicated blocks to another datanode as per rep. factor 3. So the decommission will not effect the data available in the datanode.