I write C++, and complain about still not having a portable C++17 implemenation that fully supports parallel STL, not having a full compliant C++20 compiler, embedded compilers kind of still catching up with C++14, and I could rant on.

Yes to all that, but oneapi::dpl has been out for a while.

And how much does that help anyone in systems that can't make use of it?

As much as C helps on TinyOS with a MICaz board I guess.

And what has that to do with ISO C++ compliance for portable code?

It has to do with how ridiculous it is to complain about where the afromentioned very portable library is not supported.

Are there any embedded compilers left that try to implement their own C++ frontend? To me it looks like everyone gave up on that and uses the clang/gcc/EDG frontends.

Yes, and even those that have compiler forks aren't on vLatest.

Interesting you mention EDG, as it is now famously know as being the root cause why Visual Studio development experience lags behind cl.exe, pointing out errors that compile just fine, especially if using anything related to C++20 modules.

Apparently since the modules support introduced in VS 2019, there have been other priorities on their roadmap.