16 Deleted Comic Book Movie Scenes You're Not Allowed To See
12. The Wedding - Suicide Squad
Thanks to an image released in a Chinese film magazine, it appears that there was at one point to be a scene in which Harley Quinn donned a wedding dress very much in keeping with her idiosyncratic style. Presumably, this was to be used to flesh out the backstory of her relationship with The Joker (perhaps as another part of his emotional abuse of her - leaving her at the altar).
Or maybe this was part of her romantic fantasy inspired by Enchantress towards the end? That would make sense if the rest of the fantasy didn't inherently (and tragically) imagine that their relationship was normal. So it's probably better to assume it was part of Mr J's abuse games that ultimately mostly ended up cut.
Why It Was Cut
Same reason the rest of it was cut, because the creative decisions that went into cutting the final film were inherently, irreparably broken. And the reason it wasn't included on the blu-ray as an Extra will no doubt be for the same reason the rest of the missing scenes weren't.
If you know the answer, please, do your public duty and inform us all.