So i have added a struct for an unknown byte in a packet called "unknown_MJ1d" (the MJ is short for ememjr so i can find changes i made quickly) for data versions 63119 and higher

and i have added this to my code to test it what happens if i use an older client such as dov and it gets to the above line is it basically ignored since "unknown_MJ1d" does not exist or do i need to put an if statement and only execute it when i am on version 63119 or higher

It will be ignored as it does not exist in older structs.