There have long been multiple implementations of Sass. Most notably, the canonical Ruby version, now at 3.5.6. Then there is LibSass, the C++ version, which is at version 3.4 and...
Current LibSass 3.4 should be compatible with Sass 3.4.
It's a little unfortunate LibSass isn't up-to-date with current canonical Sass, but I think it's on freeze as it's been stated that LibSass will never be canonical Sass. Update: it's not on freeze. It was actually Ruby Sass that was once on freeze with the intention of allowing LibSass to catch up. As I write, LibSass is at 3.5.2, so it's close.
Dart Sass just went 1.0.0, and is now 100% compatible with Ruby Sass 3.5.6. They announced that Ruby Sass has now begun deprecation and—after March 26th, 2019—will no longer be maintained.
The future of Dart Sass looks pretty good:
Another big announcement: as of today, I'm working full-time on @SassCSS. This has been a goal for my entire career and I'm thrilled that it's finally happening
The Dart Sass compatibility is also great, because node-sass can now switch to Dart Sass bindings and become entirely up to date. Will it? I have no idea. The maintainer of LibSass and node-sass is the same person (Michael Mifsud), and with 30+ bindings to LibSass, I can't imagine LibSass just going away. I guess we'll just have to wait and see a while. I gotta imagine someone will jump on making a node version of Dart Sass one way or another.
I, for one, would love to see a Web Worker version.