I understand /usr/local to be for anything not managed by your distribution but following the standard system layout (e.g. Python that you compiled yourself) while /opt is used for things that are (relatively) self-contained and don't integrate with the system, similar to Program Files on Windows (e.g. a lot of Java software).

Regarding "that's a Linux-ism" - well yeah? Linux is the main OS this is about. FreeBSD can do what it wants, too.