Hello @KMrunal, 

Thanks for being part of Cloudera Community. 

I will try to answer your questions: 

1. What is the exact reason for deprecating and removing InvokeGRPC in NiFi 2.x?
Answer: This was done because those processors were not really well maintened, so there was some tech debt that was not easy to solve. 
That is explained on https://issues.apache.org/jira/browse/NIFI-12069 and https://issues.apache.org/jira/browse/NIFI-12071 

2. If we take a fork of the NiFi 1.21 gRPC NAR and use it in NiFi 2.6.0, will that work?
Answer: That may work if you take the fork and rebuild it for NiFi 2.6.0. Now, this can cause problems due to dependencies, such as Netty, that is also documented here: https://issues.apache.org/jira/browse/NIFI-12367
And of course, there can be CVEs that are not solved because the processor was not properly maintained on those versions. 

3. Is there an alternative recommended approach for gRPC integration in NiFi 2.x? 
Answer: per the context, I understand you're using standalone NiFi implementation, not CFM from Cloudera. 
ExecuteScript can be used. 
There is no specific alternatives offered, and tell to use custom processors: 
https://nifi.apache.org/docs/nifi-docs/components/org.apache.nifi/nifi-grpc-nar/1.5.0/org.apache.nif... 
Checking the CDF from Cloudera, we do offer InvokeGRPC in our CFM 4.11.0: 
https://docs.cloudera.com/cfm/4.11.0/release-notes/topics/cfm-supported-processors.html
That version includes NiFi 2.4.0. 
So that could be an option if you use CDF.