Why does iStat Menus disable S.M.A.R.T. on my Mac?
Why did iStat Menus disable S.M.A.R.T.?


Nothing can disable S.M.A.R.T.  It is a technology built into the firmware of your HDD or SDD that reports on the health of your drive.  It's not something that can be disabled. 


  After I installed iStat Menus my launchd job immediately threw me a
  popup saying me to backup all my files. I checked with Disk Utility
  and I saw S.M.A.R.T. status as Unsupported.


This is because there is a system service residing at the hardware layer that's intercepting responses from the drive's S.M.A.R.T. reporting.  Disk Utility is attempting to send a request to the drive to report back on it's status and iStat Menu is (for a lack of a better word) grabbing the result; no result comes back - thus it's "Unsupported."

Regarding your script....

While your script in and of itself is good, Disk Util's "S.M.A.R.T. Status = Verified" is nothing more than the "check engine light" on a car's dashboard.  It tells you something is wrong, but not what and only  after something has failed.  This is about as useful as closing the barn door after the horse escaped.

S.M.A.R.T. has a number of attributes that Disk Util doesn't monitor nor report on.  To get comprehensive reporting and more granular data as well as thresholds that will warn you of impending failures, look to tools specifically made for this type of task.  


Smartmontools (Free, CLI based)
Disk Drill.  (Free for monitoring, paid for data recovery)
Drive Dx.  (Free trial, paid for continued use)


I have personally used the built-in Disk Util to get the S.M.A.R.T. status in a pinch.  However, I  prefer the "pro" tools designed specifically for this type of task.  Smartmontools is sufficient for me, but for clients who aren't comfortable with a CLI, I recommend either Disk Drill or Drive Dx