The fact that Izzy and Meliorn are in a happy, healthy and non-monogamous relationship - and it’s totally normalized.
Like Izzy asks right at the beginning if she can trust him not to fool around with anyone else but immediately laughs and says “just kidding” (which to me, suggests they’ve already discussed this kind of thing and agreed that they’re cool with seeing other people). And yeah this could be a defensive thing but honestly as far as I can see she seems genuinely okay with it?
Like when they discuss the fact that Meliorn is currently in a sexual relationship with Camille as well, Izzy’s not even really interested in why he’s seeing Camille (she even says “I totally get it”). It’s clear that she’s just interested in how the relationship might help her to rescue Simon.
In addition to the fact that Meliorn is seeing both Izzy and Camille, Izzy regularly and openly flirts with other people (Simon, Clary, those demon dudes in the first episode) and no one ever says “But I thought you were with Meliorn?” or “What would Meliorn think about this?” even though everybody knows about the relationship.
Meliorn isn’t presented as being a bad person for seeing two girls at the same time and Izzy doesn’t come across as even slightly jealous or possessive. Her and Camille don’t get along but that’s because of totally unrelated things like: Camille kidnapping and killing her friends, Camille stealing important Shadowhunter artifacts, Camille trying to steal her brother’s boyfriend - and so on. In any other show their confrontations would have involved some comment from Camille about how Meliorn likes her better or is going to break up with Izzy to be with her, but it’s not a point of conflict between them at all.
What’s even nicer is that Meliorn and Izzy’s relationship is still shown to be very loving and meaningful, despite not being monogamous. She risks everything to save his life and their reunion when she succeeds is very emotional. But there’s no dramatic declaration of love, no “now I see that you’re the only one for me” because they don’t need it. They already know where they stand with each other and monogamy has nothing to do with it.
The Scholomance was a piece of Shadowhunter history come to life. A cold castle of towers and corridors carved into the side of a mountain in the Carpathians, it had existed for centuries as a place where the most elite of Shadowhunters were trained to deal with the double menaces of demons and Downworlders. It had been closed when the first Accords were signed: a show of faith that Downworlders and Shadowhunters were no longer at war.