What does the Sectumsempra spell do?
Why did Ron break up with lavender?
Did Ron actually like lavender?
Who was Hermione’s first boyfriend?
The reason it wasn’t classified as an Unforgivable Curse is because the Ministry wouldn’t have known about it – they would be unable to make any decision on something they don’t know exists. That, by itself, doesn’t say whether the Ministry would classify it as one if they knew about it.
Why did Harry use Sectumsempra on Draco?
Sectumsempra was a curse invented by Professor Severus Snape that lacerates the target and causes severe haemorrhaging. Snape created it as a student of Hogwarts, with the intention of using it against his enemies, likely including the Marauders, and it became one of his specialities.