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Yes, you have to buy the Steam version.

“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.

https://steamcommunity.com/app/648800/discussions/0/3223871682627386165/