They're just not really needed now that their core features are provided by other parts of the stack. Variables are stable in CSS itself now, modularisation is done as part of DOM management with React etc, and composition is either done in code with JS or provided by something like CSS modules 'composes' property.

You can still use a preprocessor like SASS with that stuff, but it's just another layer of complexity for not a whole lot of value outside of familiarity.