To summarize, the dusk elves (and really, one dusk elf in particular) earned Strahd’s wrath, so in retribution he set out to exterminate the entire race. But he wanted to extend their torment, so he had all of the female dusk elves killed, but left all the men alive. With their long elven life spans, they were stuck slowly going extinct over hundreds of years, knowing that with no women, they couldn’t reproduce.
Personally I don’t find that particularly upsetting, though I could see it meriting a content warning for sure. Mostly it shows Strahd’s cruelty and misogyny, which I don’t think is a bad thing as long as your players are ok with the subjects of genocide and violence against women being broached in the game.
Where it does get kind of icky for me though is that the particular dusk elf who pissed Strahd off so much is an NPC in the adventure, and one of the player’s potential prophesied allies against Strahd. His goal is to go to the amber temple, where a bunch of ancient evil vestiges are trapped, so that he can make a deal with one of them to resurrect his sister, to save his race. And since their doom is caused by the inability to reproduce... the implication is that the dude wants to resurrect his sister to become a literal baby factory. Also, if you succeed in resurrecting the sister, it turns out she’s evil and in love with Strahd.