The best manner you may currently get Rocket League on Steam is Rocket League Items by way of buying a key from a third-birthday party key reseller. Many of these websites can be ethically questionable (to place it lightly) so we may not be linking to any here, but you can effortlessly find them with any seek engine.
Another workaround you could do is set up Rocket League from Epic Games, after which upload it to Steam as a non-Steam game. This can be carried out through beginning Steam, choosing "Add a Game" from the lower left corner, clicking "Add a Non-Steam Game", after which finding Rocket League inside the document browser. You won't be able to music playtime or LOLGA.COM achievements with this method unfortunately, however it enables you keep your games centralised to Steam if that is what you want.
