Rocket League is free to play as of today, and can be acquired on the Epic Games Store. Even if you don't want to play Rocket League, you might want to add it to your library: Doing so before October 23 at 11 am ET will get you a $10 Epic Store coupon to spend on another game.
As Psyonix announced back in July, the catch with the free-to-play update is that Rocket League is no longer available on Steam for new PC players, who must now pick it up on the Epic Store. Existing Steam players can keep using the Steam version, and it'll continue to get updates. As of last week, though, you do need to link your Steam version to an Epic account.
Along with the free to play update, a new competitive season has begun, which Psyonix is calling Season One—we're starting over. The new competitive tournaments are also live, and a new Rocket Pass is available. If you're using the Steam version of Rocket League, you can still use your Steam Wallet to buy the Credits you need to upgrade to the premium pass, or to buy things from the Item Shop. (I know because I lack self control and bought the Rocket Pass myself, even though I don't really like the items in this one.)
You can see the full patch notes for today's update here. An update that went out last week also brought some big changes, including tweaks to the UI that anyone who's been using the old menus for years will almost definitely dislike (why is everything in italics now?).
If you already have Rocket League on Steam and have linked it to your Epic Account, you can still get the $10 Epic Store coupon by grabbing the Epic Store version. Once you have the coupon, it'll expire if you don't use it before November 1 at 2:59 am Eastern. It can only be used on purchases of at least $14.99, so you'll have to spend $5 to get the $10 off—classic coupon chicanery—but it's not as if there isn't anything worth $5 on the Epic Store.