Can we use the special parts of previous years game elements? e.g. the Hexagonal plates, the square plate that was used in the zone boundary etc?
I believe you might mean the octagonal plates on the purple dispenser, not the hexagonal, and the plates that make up the fence wall. Everything except the PVC pipe and the disks is legal for competition builds.
Each season they release a Vex IQ Legal Parts List. Below is a link to the 22-23 version; I have not found one for this year yet.
Go to Competition Overview - Competition - IQ - VEX Robotics
Open the game manual from the link at the bottom of that page, which will take you to the pdf of the latest published version, and within that document search for the link “VEX IQ Robotics Competition Legal Parts Appendix” and that gives you the latest legal parts list.
I found the legal parts list for 2023 - 24 here