Gnuffi gave you the wording: “it will remain on sale until then” until means not afterwards
you yourself also gave the wording:
this means if you own it before they remove it from the Steam store, once they remove it u will not be able to buy it and will hence no longer be able to play it on Steam unless you previously owned it
the proof that this is what it means is that otherwise there would be no need to specify it, as you can NEVER play a game on Steam that you don’t own (or have another type of access to)
Their original statement:
The PC version of Rocket League will come to the Epic Games store in late 2019. In the meantime, it will continue to be available for purchase on Steam; thereafter it will continue to be supported on Steam for all existing purchasers.
meantime it will remain available for purchase on Steam,
thereafter it will be supported
how is that not clear as day?