All signs now suggest that Epic will remove Rocket League from steam they are dancing around confirming this or not but Epic have been pretty clear on their absolute hatred of Steam and desire to utterly destroy it.
What that means for people that already own it on steam is unclear I would assume it will stay in steam accounts for existing purchases but as to future support or dlc or even the match making and steam friends support no one knows.
Since Epic has been becoming increasingly anti consumer I can see them using this popular game as another stick to beat steam with as they are clear they don't give two fucks about consumers they only care about money and getting more devs on the unreal engine.
It's a shame that Epic have gone down this rabbit hole. When they first came out with their store and it seemed like they were going to push things forward give dev's a better cut for games offer some viable competition to Steam who have had things their own way for too long. Going from where a game being on steam was a mark of quality to letting any old crap on there, innumerable asset flips and even non games working exes.
Now they are just going around throwing comically large bags of money at game devs to lock down games to the epic store. Previously it was just upcoming new games, but with this I guess it means they can buy out anyone even an existing product