There was never option to buy the game, it was always free on itch.io. You could however voluntarily donate to help the development. And no, having donated towards the itch.io version does not grand you free copy of Steam version. So yes, you have to buy the Steam version if you want to get it. It has been on sale several times so you can wait for next sale if you don't want to pay the full price. Last time it was on sale was during the Lunar New Year Sale in January with 20% discount. It'll probably be again on sale during the Steam Summer sale at the latest.
“Will players who have played or donated on Itch.io receive a free copy of Raft on Steam?”
We very much appreciate all the support and feedback we have gotten from our community, however, playing or donating to the itch prototype will not grant access to the Steam version of Raft.