It seems so obvious, but I feel like script writers should be given a reliable way to tell if the local machine joined a mission that was already in-progress.
I've seen many methods involving time/checking if the player isNull. However as far as I know they all have potential to return a false positive/negative depending on timing of the script and ping/etc.
So currently there is no 100% reliable way to tell if the local machine is a JIP player (especially since all methods I know of currently rely on your code running as soon as the players joins the mission - which means if multiple scripted systems need that information you have a bunch of stuff running at once to set some variable for later on).
The way I envision this command working is as follows:
"Returns true/false based on whether the local machine joined the mission already in progress. Always false on the server and in single-player."
This is really a request of convenience more than anything. The above methods do work the majority of the time, but with a command like this we could access the information at any point during the game and be confident that it will always be accurate.