trap 'echo "unkillable"' KILL
Not applicable. This is not possible.
SIGKILL and SIGSTOP can not be caught/ignored (according to POSIX and as implemented on platforms including Linux and FreeBSD). Trying to trap this signal has undefined results.
None. If you come across one, please file an issue about it.
ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.