As we mentioned in the introductory blog post for Unity 2018.3, the old .NET 3.5 runtime has been deprecated. What does this mean? Any new projects in 2018.3 will use the new .NET 4.x scripting runtime by default. The old .NET 3.5 scripting runtime is still an option for new projects in 2018.3. The scripting runtime will not change for any existing projects upgraded from earlier Unity versions. The Unity 2018 LTS release will continue to support the .NET 3.5 scripting runtime for its lifetime. This will be the final Unity LTS release to support the .NET 3.5 scripting runtime. We will be removing the .NET 3.5 runtime in the 2019.x series of releases (although not in 2019.1). Please let me know if you have any questions about the scripting runtime versions. Thanks!