Harley and Ivy have had an on-again off-again thing going since the late 90's or mid 00's. It's been at least decade since Harley ditched Joker in the mainline comics and moved on to greener pastures.
They actually first implied it during the Batman: The Animated Series, as a "Sappho and her friend" style relationship, only coming out and saying it later in the comics.
Harley is bisexual, a demographic which often gets erased the moment they're not actively flirting with multiple people of different genders at the same exact time.
I put off watching that for a long time because I didn't really like the Harley Quinn character and her obsession with Joker and that whole abusive relationship they had, compounded by all the people who treated Harley and Joker like it was a relationship goal.
But I was very pleasantly surprised when I did eventually give it a shot. Yeah, it does include some Harley and J stuff, but they kinda had to because of how ingrained that relationship was into Harley's pop culture identity. But it is done well. The series is one of my favorite in the Batman mythos.
It's also kinda interesting how Ivy seems to be holding up better morally than Batman himself does. She's an environmental warrior while he's a status quo warrior, and that billionaire side of him holds up less and less well as it becomes more and more apparent that even billionaire philanthropists are really just taking credit for giving away wealth they shouldn't have had in the first place.