The Scripting Runtime Version is currently set to .NET 3.5 by default on new projects. As of 2018.3 this should be .NET 4.x by default.

Seems to have been changed in 2019.1.0a3.

1. Install Unity
2. Launch Unity
3. Create a new project with the 3D or 2D template
4. When the project opens, go to Edit > Project Settings > Player
5. Scroll down to Configuration
6. Look at the Scripting Runtime Version

Expected Result:
The Scripting Runtime Version should be 4.x by default.

Actual Result:
The Scripting Runtime Version is 3.5.

Tested on Windows.

Occurs on:
2019.1.0a3. 2019.1.0a5, 2019.1.0a12

Does not occur on:
2018.3.0f2, 2019.1.0a1, 2019.1.0a2