> Nowadays that compatibility is up to C11 subset.
Not true unfortunately, the "C subset" is still stuck at something that can at best be called a fork of "C95" which was then developed into a "bastard language" that resembled C on the surface, but isn't actually C (e.g. the incomplete designated init support in C++20 is the best example of this half-assed "looks like C, but isn't actually C" philosophy).
Since 1993, I never saw any need to keep bothering with C other than having it imposed on me, C++ had enough C89 subset on it, if I ever miss coding like C and its warts.
Nowadays that compatibility is up to C11 subset.