RCTgo

There is nothing that specifically states that the competition is strictly for roller coasters, and several users in the past have entered non-coasters, most commonly with water rides. Two of these builders succeeded and won.

patricio6143, with his whitewater rapids ride, was the first to win without building a roller coaster (despite what he filled in under "Type")...

https://rctgo.com/downloads/view/heart-o...mazon.1961

...while AzoRctMan holds the honor of being the only player to ever win Build It! with a log flume.

https://rctgo.com/downloads/view/jungle-log-jam.3781

The opening post is as old as the competition itself, and until Round 41, it was known as the Featured Coaster Competition. This does imply that, at one point, it was strictly for roller coasters. That, plus the fact that 99% of entries are roller coasters anyway, is likely why the word "coaster" is used as a synecdoche for "ride" or "entry" in many instances in that post.