This thread is irking me beyond....... words.I don't know where to even start.
Thatch to me means: "partially decayed organic mat does a horrible job at retaining water".
Anything below this should be soil.
Above it you have your grass, and dying/dead crowns.
Its the dying dead crowns that look like the color of wood that I would leave for all the benefits that smallaxe has stated.
That thatch layer could be bad for seed if it is too thick because when the seed roots into it it does not offer the same protection that soil will. Unprotected roots will die in a short time and so will all grass you just seeded.
Measure the thatch and 1/2" or less is fine.
The OP asked how often to de-thatch. Answer is: As often as necessary to maintain a healthy level of thatch.
I have too customers side by side:
first guy has 1/2" of thatch, Lawn looks good all season.
Second guy has no measurable thatch, lawn looks great in the spring but then I am skipping mows half way through the season because he has no thatch the soil has no protection to the sun.
Both house are mowed the exact same day by the same mower by the same guy guy using the same fertilizer.
Another guy a few blocks away has alot of thatch, 2" and although his lawn looks good all year from the road, a birds eye view tells a different story. There are many areas where there are complete dead spots. Dig below the thatch and soil is bone dry.
So am I right about what thatch is?