You are correct J Body.

Some trucks (diesel I believe) did have an additional sensor in the line - for one reason and one reason only.

They get driven hard for LONG periods of time and to protect the trans from sustained over temp they force the trans down in to 3rd gear and then force lock up so you slow down and cool the fluid IN the trans.

The chart included above is missing one explanation.

It’s called “time at temperature”.

Going to 300 degrees for 5 seconds then dropping to 200 a hundreds times will cause very little fluid degradation. What kills the fluid and the seals is keeping it at 300 for extended periods of time.