I don’t integrate with Steam, so I don’t know what their API allows for. Companies that handle access control within their own game/launcher will have an easier time as all of that will be handled by themselves, and those that rely on a third party such as Steam will of course be limited to however Steam allows key distribution.
For example, if you was to release the beta for your game on steam for everyone to access, but then inside that game you could do whatever checks you like to see if the user has redeemed the Twitch Drop Reward granting access to actually play the beta. That could be one option. If on the other hand you wish for people to not be able to even install the game without having redeemed the reward then yes you will need to find a way to get beta keys to the end user, or see if Steam has an API endpoint to add a game into a users library.
Whatever way you choose is entirely up to you to decide what is best for your product as it is beyond the scope of Twitch Drops to handle the distribution of rewards.