runtime JAVA_VERSION vs. Java build target

I've run into this recently:

when I build a Java port, generated JAR will be built
with a compiler set by JAVA_VERSION which is our runtime setting.

For example, when building with OpenJDK 11, and later
running something with env JAVA_VERSION=8 ... I will
get class file version errors.

Since textproc/xincluder is a small and obsolete
piece of software, I have decided to tell javac
to target Java 8 in all cases[1].

Shall this be a general practice? If someone
decides to set JAVA_VERSION high and rebuild
Java dependencies from source (those which can be rebuilt)
- JAVA_VERSION cannot be really used to switch at runtime.

On the other hand, one might the code to be optimized better.

What is the best practice here? Shall we keep JAR libraries
that can be used as dependencies compatible with the oldest
Java runtime we have?

Marcin

[1] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=272559