FWIW, in my project the main reason we don't issue security advisories for "unsupported" code ("experimenal" or "tech preview") is to reduce the burden for our downstreams: many of our immediate downstreams are expected by their users to apply every single security patch, regardless of whether they even use the affected functionality. For cloud providers doing this across a massive fleet, this is a fair amount of work that's worth avoiding if we can.
On the other hand, since the definition of "supported" is specifically designed to help downstreams, if it were known that some bit of code was widely used in production, we'd be open to declaring it "security supported", regardless of whether we thought it was "finished" or not.