I don't think this is really covered by FHS. As I read it, FHS doesn't say anything about whether shared library files should or should not have the executable permission bits set. It only talks about whether executable programs should be installed under /usr/lib or elsewhere. .oct files are clearly not programs, but are dynamic modules, so they are allowed to be installed under /usr/lib.

This is more a question of Debian Policy §8.1, which says

> Shared libraries should not be installed executable, since the dynamic linker does not require this and trying to execute a shared library usually results in a core dump.

but also says

> Shared libraries […] that are only loaded as dynamic modules are not covered by this section and are not subject to its requirements.

There doesn't seem to be a clear right answer here, and the easiest and most consistent thing would be to continue to install .oct files with the executable permission bits set. But we could discuss whether we should change the permissions on .oct files, either when building or when installing them.

We should also consider .oct files built by Octave's own build system as well as .oct files built by mkoctfile and installed by the pkg command equivalently.