How is the optimization implemented? How are current B-Tree algorithms altered by this optimization? In what circumstances would I choose not to deploy this option?
A patent for a reverse key approach.
I had a quick look using DBCC PAGE, comparing 2017 to 2019 and 2019 with and without OPTIMIZE_FOR_SEQUENTIAL_KEY on an unique clustered index for an int IDENTITY column. There was nothing that obviously explained the new behaviour. This makes me think its an algorithmic thing, rather than a structural thing, which makes sense.
A blog post from MS.
This feature seems to centre on the detection and avoidance of convoys.