Is it possible that players would be allowed to post own warzone extensions? currently it is forbidden but i have seen many cool APIs made by folks out here so would gladly see it here. like this multiuser script

You can post userscripts and userstyles as well as mods that use the official mod API system. Not really sure about anything else.

i have seen ppl modifying ui for example, best one adding last seen to tournaments. this is currently forbidden

i think anyone can make mods im not sure 100%