Well, technically, it is possible to uninstall A9LH with a sysNAND update.
If you use a crappy CFW like Gateway, it won't have FIRM protection. What this means is that FIRM0 and FIRM1, which are where A9LH resides, won't be protected from updates, and the update will overwrite it. Due to the way A9LH works, it'd be a miracle not to brick from this.
But if you stick to Luma, you won't have a problem Luma has the necessary patches to make sure this doesn't happen.