Background:
This feature requests comes from the wish of having such feature in toolchains_llvm (see bazel-contrib/toolchains_llvm#591). After taking a look on how this could be implemented and what it would make sense for the overall community, I reached the conclusion that it does not make a lot of sense that a specific toolchain supports that if rules_cc does not support it.
A use case on why this would be needed can be found in the original toolchains_llvm issue but I will put it here again for completness.

Use case:
There are situations where it is helpful to implement a toolchain feature for all toolchains that your project supports.
In our case, this was used for implementing different warning levels/groups that is compiler agnostic.
That allows us to have something like:

cc_library(
    name = "foo",
    srcs = ["foo.cpp"],
    hdrs = ["foo.h"],
    features = [
        "safety_related_warnings",
    ],
)

cc_library(
    name = "bar",
    srcs = ["bar.cpp"],
    hdrs = ["bar.h"],
    features = [
        "security_related_warnings",
    ],
)

Request:
The main question for the maintainers would be, are you interested in that feature? Do you see this as something that we could add to rules_cc? If yes, do you think that it should only supported with the rules-based toolchain approach or also with the old approach? Right now I would say both because afaik the old toolchain approach is not yet replaceable by the rules-based approach in all use cases (i.e. #434).
I have the capacity to work on the topic, I would just ask before I start putting too much time to know the interest and if there is any specific request on direction.

Thanks!