libsass is being deprecated according to sass/libsass#3123, because they don't have the resources to keep it uptodate with the sass language.

For now, Scssphp is less compliant than libsass AFAICT, and does not support any of these new language features.

I can see several potential futures for this library:

1. stay a pure-PHP implementation of the old sass language features (i.e. status quo). This will make the package less usable over time, as it won't support modern sass code. See Support for sass modules #55
2. implement a pure PHP implementation of the modern sass language features. This should probably involve some full rewrite by porting dart-sass rather than being done incrementally, as it would require a major rewrite anyway to support modules AFAICT. This solution would require lots of resources.
3. implement a PHP wrapper around dart-sass using the Sass embedded protocol and potentially a custom composer installer to install dart-sass. This would make the library dependant on being able to run a sub-process, which might not be available in shared hostings. https://github.com/sass/embedded-host-node could be used as an inspiration once it is implemented (currently WIP)
4. abandon the library