[x] I wouldn't include a users index route. Reason being, the news feed is essentially a list of checkins sorted by the created_at date of the checkin itself. While your checkins index should probably return user data with it, I don't think you need a users index.
[x] Which route returns all the checkins for a particular drink / brewery? Would make sense to include with the drink / brewery show pages (if so, I'd mention that explicitly), but could also be nested checkin index routes (example: /api/drinks/1/checkins could retrieve all checkins associated with a drink);
[x] ^ Same goes for getting all the drinks associated with a particular brewery, all the checkins made by a user, and all the drinks reviewed by a user (if you include that feature in your user profile: see last MVP note)
[x] These probably don't warrant their own routes, but I'd also be explicit about which routes return data for the comments / toasts associated with each checkin
To Revise:
usersindex route. Reason being, the news feed is essentially a list of checkins sorted by the created_at date of the checkin itself. While your checkins index should probably return user data with it, I don't think you need ausersindex./api/drinks/1/checkinscould retrieve all checkins associated with a drink);