Not sure what you mean. FIDL as a language and protocol is conceptually inherently cross platform. There are already bindings for multiple language platforms that can be generated from a FIDL specification, and theoretically one could implement a FIDL service anywhere. That said, FIDL services in the system provide a sort of ABI -- the F in FIDL stands for Fuchsia, after all -- and I'm not aware of any actual efforts to implement these on platforms that aren't Fuchsia.
We use FIDL on host (Linux & macos) platforms via the overnet project. As far as language ecosystems go, as swetland rightly points out below, being runtime agnostic is important to us. FIDL being easy to implement generators for new runtimes is part of that. There is some nascent documentation on porting runtimes here: https://fuchsia.dev/fuchsia-src/development/languages/new?hl...