After upgrade from 2018.2.17 to 2018.3 my game is destroyed

I have upgraded unity from 2018.2.19 to 2018.3 and now my game is destroyed. But I have created a backup before upgrading unity and my project.
After Update I have got worse lighting, my textures are too darken, and I get a lot of errors, post process volume says IsObjectInCamera is not valid in Scenemanager and and... I have comented out some errors and now it looks very bad. Some particle effects are resized or rotated. Shadow looks very bad and and
what is wrong with unity 2018.3? 2018.2.19 was fine.

is unity 2018.3 required for future projects or can I continue developing my game with 2018.2 and publish it?

 

No, it's not required. It's never been required.

 

The first rule of video game club is - we don't upgrade to new platforms mid-project.