I lost the squiggly red underline feature

If your want to add/remove C/C++ underline squiggles, try this:

in javascript I solved this by going to Tools > Options > TextEditor > Javascript > advanced > Uncheck Show Syntax Errors.

Add-ins often turn off the Visual Studio features so they can replace them. And of course don't restore them when you uninstall them, so you're pretty much forced to keep using them if you can't figure out how to undo the setting changes.

Tools + Import and Export, Reset might be your best shot. As long as you haven't customized VS yourself.

The specific setting is Tools + Options, Text Editor, C#, Advanced, "Underline errors in editor" check box.