Alex Gavrilov
August 11, 2019 
My short practical guide on how to stop nasty spam of obsolete warning messages in Unity game engine editor. We need to find a way to disable those warnings because in most cases they are just useless.
I don’t know why, but Unity game engine team likes to change names of classes and their methods pretty often in new Unity versions. Even if those classes and methods do exactly the same as before, in the new Unity version they may have a new more beautiful name, so the old name becomes obsolete. This is a real problem if you use assets from Unity Asset Store because publishers often do not care about those obsolete warnings at all. So we need to find a way to avoid them and pay our attention to more important warning messages.
We can’t disable obsolete warnings in Unity game engine editor directly because there is no such option for that. However, there is a simple trick how you can do it by adding a small text file to your PROJECT_NAME/Assets directory. Just open any text editor you like and create a new file with just two lines:
0612 code is for simple obsolete attributes like this ->>> more info.
0618 code is for more advanced obsolete attributes like this ->>> more info.
Please note that when targeting the .NET 3.5 Equivalent (default for Unity 2018) scripting runtime version, file name should be "mcs.rsp". But when targeting the .NET 4.x Equivalent (default for Unity 2019) scripting runtime version, file name should be "csc.rsp".
You can check the actual .NET scripting runtime version in Unity Project Settings > Player > Other Settings > Scripting Runtime Version. Just save this text file with correct name and place it into PROJECT_NAME/Assets directory.
That’s all. Now you can concentrate your best efforts on making an awesome game, not reading nasty spam of obsolete warning messages all the time.