List encrypted RDS snapshots using CLI
Either use the --query attribute

aws rds describe-db-snapshots --query "DBSnapshots[?Encrypted].DBSnapshotIdentifier"


If the [?Encrypted] does not work, try [?Encrypted == 'true'] as the first is from JMESPath improved filters. The quotes might need to be backticks in some case. It all depends.

Second way:

aws rds describe-db-snapshots --output json | jq '.DBSnapshots | map(select(has("Encrypted"))) | .[].DBSnapshotIndetifier'


One of them should work. Might need a bit messing around as I don't have your output to tune the jq command. Feel free to edit and fix the answer.