I have some more update. Instead of making one big change to support both block scoped stage as well as replace FlowGraphWalker by Fork scanner - it brings optimization to scanning process. Later is considerable amount of work as it requires complete re-write.
Basically this ticket only adds support for block scoped stages. task/
JENKINS-37323 is ready with this support as soon as pipeline-stage-step gets released.
I am going to open another ticket to replace FlowGraphWalker by ForkedScanner.