As of NiFi 1.21.0 PutLambda doesn't allow to to and fields (see pictures in attachments). The lack of this feature complicates our workflows because it requires to have each processor assigned to specific lambda and if the flow has a lot of lambdas, that becomes a mess really quickly.
By allowing to use nifi expression language in & fields, it simplifies certain nifi flows and makes better nifi usage experience.
In attachment, I add some code examples of how this change could be implemented. Be aware that these examples contains some additional changes for NIFI-11533 as well.