I can't imagine why a concern about a slow engine failure.
If the pilot flys the airplane by SOP then a gage (bell horn, color change, etc.) warning on the engine is sufficient.
The problems that have occurred in flight is where a slow thrust loss goes unnoticed until it eventually results in an aircraft upset because the pilot was not monitoring the more critical flight instruments.
Could be I don't understand the question (spoken words are so much better
