This page lists any changes in 2018.3 which might affect existing projects when you upgrade from earlier versions of Unity
flex-basis. The flex-shrink and flex-basis parameters are optional. If omitted, flex-basis defaults to 0 and flex-shrink defaults to 1.
flex: Nwas equivalent to
flex N 0 auto. This now follows the CSS standard by making it equivalent to
flex: N 1 0. To preserve the old semantic, you should replace all
flex: Ndirectives with
flex: N 0 autoin your USS files.
Physics behaviour has changed and some Projects may behave differently with the new version. In particular:
Unity 2018.3 includes some particle bugs fixes and this can affect your projects that were created in a previous version.
Before 2018.3, the Unity Editor used the mono C# compiler (mcs) when compiling C# files in a project. From 2018.3 onwards, the Roslyn C# compiler (csc) is used for projects targeting the new scripting runtime (.NET 4.x Equivalent). Different behavior may be noticed from the switch to Roslyn:
csc.rsp. See PlatformDependentCompilation.
UnityScript (.js) and Boo (.boo) script files can no longer be compiled in the Editor.