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I just spent 1/2 hour trying to figure out why I couldn't get DELETE to show up active when I was trying to delete an interface under the new UI. (Which btw, this is seriously over-complicating how to delete btw)
I ended up deleted elements instead of deleting the interface because it is so confusing.
Sorry to hear about your experience attempting to delete an interface. We definitely don't want to make things more complicated for our users! I've shared this feedback with our ProdDev team. I'm glad you figured out how to delete an interface, but for anyone else having trouble with this, feel free to check out this support article for instructions on how to delete interfaces.