Blcli can do the decommission.
one by one? it's time-consuming. Any other way, is there something like an undocumented bulkDecommission?
Use the performance commands and a for loop
If there's a huge number of servers involved, one idea (I haven't tried this) might be.
Use the 'Server listAllServers' command for each environment and then do run 2 'diff' commands on the outputs.
One showing those servers in Prod but not in Dev (New servers), the other showing those in Dev that aren't in Production (decommissioned).
Then you could just run a loop for each list.
A little more time consuming to create initially but should greatly reduce the number of imports and decommisions.