$ echo -e "# shellcheck disable=SC2148\n:" >test
$ shellcheck test

In test line 1:
# shellcheck disable=SC2148
^-- SC2148: Include a shebang (#!) to specify the shell.

It's even colored red as an error, not a warning. No, I don't want a shebang - some scripts are only meant to be sourced, and should not be executed on their own. In fact, it would be nice if disable=SC2148 could be placed anywhere in the file, so the first line can be reserved for comments.