Plugin fails to build when strict include checks are enabled. This can be easily tested with UAT command

.\RunUAT.bat BuildPlugin -Plugin="{workspacePath}\Engine\Plugins\SocketIOClient\SocketIOClient.uplugin" -StrictIncludes -Package="{workspacePath}\LocalBuilds"

What -StrictIncludes does is adding -DisableUnity -NoPCH -NoSharedPCH to UBT when compiling. This ensures that each code file has all the needed includes and therefore is of a good quality on that side.

Will create a pull request that fixes some of the issues.