We've all been duped.
A harem anime is a romantic comedy where the lead has the attention of more than two girls or boys, each candidate having their own personality and set of fans who cheer them on, nothing gets an anime fanbase so riled up as having ships to support, just ask Ash Ketchum.
But you never had a choice, more often than not it's usually the first girl who wins.
Why that's so annoying for a watching crowd is that they are also the least popular choice; Naru Narusegawa is hated among the fanbase and is used as the poster child for hatred of tsunderes.
I don't think much has changed, in this era, an anime literally has to make the correct choice or face a public execution, although the Gundam Wing yaoi baiting was just pathetic back in the day, but we still get it now, and no Midoriya will never love Bakugo.
Some anime with dating game backgrounds will use the option of pick everyone, Photo Kano gave us time and endings with each girl and is left up to the fans to decide which one should be canon, Happy Lesson flat out has the lead marry everyone, spare a thought for anime that never have a conclusion like Sakura Taisen despite us leading to believe there would be one.
So choice in what girl or boy to have is entirely at the writers digression.
The only choice from an actual harem to be correct is from Kiss Him Not Me.