Google has added the option to create widescreen presentations to its Slides presentation webapp. Indeed, Google appears so enamoured of the idea it has now made 16:9 widescreen the default option for all new presentations.
If you want a different ratio (useful if, for example, you know you'll be working with an older projector which doesn't have widescreen output), you can change the format by choose File --> Page setup. You can choose 4:3, 16:9, 16:10 or a custom size.
This is a useful addition, but also reminds us that Google's web-based office apps can lag behind their desktop rivals. Microsoft's PowerPoint has offered a widescreen mode since 2007, and made widescreen the default in the 2013 release.
More magic in Slides: editable themes and widescreen presentations [Google Drive Blog]