Unfortunately my memory of this isn't very good (I never had much involvement with the 737, so didn't pay as close of attention to the discussions as I might have). But after the China Southern 3943 crash (one thrust lever stalled at idle, resultant thrust asymmetry resulted in a loss of control and crash), Boeing did come up with a warning that there was excessive thrust asymmetry with the autothrottle active (I was on the Boeing Propulsion Safety Committee and it was discussed during at least one of our meetings). I don't recall the detains of how it worked or what it did, but I had assumed that - having contributed to a crash - it would be fitted fleet wide (by AD if necessary). For whatever reason, it apparently wasn't...