It is possible to install a handler for most signals, and that handler can be configured to ignore the signal.

Signal 9 cannot be ignored.

I don't think of 9 as really being a signal to the process at all, more of an instruction to the OS kernel to terminate the process

Lots of commandline tools will hold on to dear life except for the sigkill. I often have this with running background tasks which get one of their threads in an infinite loop or wait state.