I really respect that USE_LANGUAGES is confusingly named, and misunderstood by many pkgsrc developers as a result (including clearly some senior developers) -- it should really be WRAP_LANGUAGES and it does not pull in any kind of dependency.
Forgive the brevity, please, it's difficult for me to respond rn
What you're describing is how jlam implemented it, but over time, USE-LANGUAGES has come to also denote the c++ standard, as the language evolves, whether we like it or not. Happens for C too. Other people rely on the information in there, which is why I asked for discussion first
If all you want is a straight "c++" from "c++17" that's quite achievable with make(1) regex matching and manipulation
Tying everything via GCC-REQD to a specific compiler implementation also seems backwards. And judging by the innovation in other compilers, the wrong horse to back as well
Most times I just want to build a package from source to try it out. I want the pkgsrc infrastructure to build and install the correct tools to allow me to do so.