Slate.EnableFontAntiAliasing still exists in the latest UE5 code, and from what I can tell still sets the font rendering to monochromatic mode.
It did seem to work when I tested it locally by passing -dcpvar=Slate.EnableFontAntiAliasing=0 to my editor.
You can also set that by adding this to your project’s Editor.ini:
[ConsoleVariables]
Slate.EnableFontAntiAliasing=0