What is the legality of using PROS (Purdue Robotics Operating System) on the new v5 cortex, and in the new game? Thanks in advance.
Perfectly legal, why it shouldn’t be?
PROS has been, and always will be, legal for use in every VEX game.
And, please, it’s “V5 brain” not “V5 Cortex.”
Its legal, but it’s not supported so if you have a problem with using PROS, you might have trouble getting help. Use it anyway because it’s probably going to be better than VCS.
It’s not officially supported by VEX, that doesn’t mean there isn’t a community of people that use PROS that would be willing to help (there is).
Additionally about PROS, their documentation (that’s found on their website) is very helpful. It makes learning the language much easier, especially after you have the basics.
I haven’t used CodingStudio with v5 yet, but PROS is a really cool system and I would highly recommend that you check it out.
Actually, it is “V5 Robot Brain” and not “V5 Brain” 
It is supported for the V5 system.
Even though it isn’t “supported” by VEX, considering VEX worked with the PROS team during the development of V5, I’d say that VEX is definite “supporting” it, if that makes any sense.
Bit of a miscommunication with the meaning of “Supported”,
https://www.vexrobotics.com/vexedr/resources/programming
At the bottom of the page its listed as an option but it’s not an officially supported option meaning that you can use it but they don’t recommend it for some reason.
It’s not that they don’t recommend it per se*. You just won’t get any official support from VEX for it. Basically, if you decide to use PROS and ask about it in the Official Tech Support channel (or equivalent), you are not guaranteed a helpful answer – you may well get a canned response.
*One of the reasons for including the note is to discourage complete novices (that may have trouble with RobotC, let alone PROS) from trying to use PROS. Why is this important? Because VEX needs to maintain their brand image as a good educational robotics platform. Without the note, many would imply that VEX, by linking PROS on their website, at least supports, if not endorses, PROS. This would force VEX to either pay entirely out of their own pocket to support PROS or risk their brand image by failing to [provide means to] educate users about PROS.
VEX is supporting the existence and, to a lesser degree, development of PROS. VEX is not guaranteeing technical support availability for PROS, which is what support means in the context of the statement on the VEX website.
Ok, I wanted to make that distinction clear. VEX has been involved with PROS throughout their development cycle. But yes, they don’t recommend it because they are also the ones selling VCS 