Why is the $N local variable missing at the Group node?
I thought it would be easy to swap between Group node and Delete node and use the same
expression, but instead I have to use
Is there a reason behind not having “N” local variable in the Group node?
I found the answer to my own question. It's basically because everything in Houdini is moving toward VEX expressions, so I guess that's why $N is disappearing from some of the modern nodes.
Please correct me if I'm wrong
You've answered your own question correctly but the change isn't an arbitrary decision.
For a great overview of the changes to SOP's and the reasons for the changes check out this Webinar with Jeff Wagner.