How to delete an app from iTunesConnect / App Store Connect
Here's the answer to my question I got back from Apple support.
Hi XXX,
I am following up with you about the deletion of your app, “XXX”. Recent changes have been made to the App Delete feature. In order to delete your app from iTunes Connect, you must now have one approved version before the delete button becomes available. For more information on the recent changes, please see the "Deleting an App" section of the iTunes Connect Guide (page 96-97):
You can only delete an app from the App Store if it was previously approved (meaning has one approved version).
From iTunes Connect Developer Guide - Transferring and Deleting Apps:
Apps that have not been approved yet can’t be deleted; instead, reject the app.
As of 2016, new changes have been made to iTunes Connect. Here are the screenshots of deleting an approved app from your account.
You Can Now Delete App.
On October 4, 2018, Apple released a new update of the appstoreconnect (previously iTunesConnect).
It's now easier to manage apps you no longer need in App Store Connect by removing them from your main view in My Apps, even if they haven't been submitted for approval. You must have the Legal or Admin role to remove apps.
From the homepage, click My Apps, then choose the app you want to remove. Scroll to the Additional Information section, then click Remove App. In the dialog that appears, click Remove. You can restore a removed app at any time, as long as the app name is not currently in use by another developer.
From the homepage, click My Apps. In the upper right-hand corner, click the arrow next to All Statuses. From the drop-down menu, choose Removed Apps. Choose the app you want to restore. Scroll to the Additional Information section, then click Restore App.
You can show the removed app by clicking on all Statuses on the top right of the screen and then select Removed Apps. Thank you @Daniel for the tips.
Please note:
you can only remove apps if all versions of that app are in one of the following states: Prepare for Submission, Invalid Binary, Developer Rejected, Rejected, Metadata Rejected, Developer, Removed from Sale.
Edit December 2018: Apple seem to have finally added a button for removing the app in certain situations, including apps that never went on sale (thanks to @iwill for pointing that out), basically making the below answer irrelevant.
Edit: turns out the deleted apps still appear in Xcode -> Organizer -> Archives and there is no way to delete them from there even if there are no archives! So more looks like a fake delete of sorts.
Currently (Edit: as of July 2016) there is no way of deleting your app if it never went on sale.
However, all information except for SKU can be edited and thus reused for a new app, including the app name, Bundle ID, icon, etc etc. Because SKU can be anything (some people say they use numbers 1, 2, 3 for example) then it shouldn't be a big deal to use something unrelated for your new app.
(Honestly though I'm hoping Apple will fix this soon. I almost hear some Apple devs finding excuses for not implementing it (you know, it will break the database and will kill innocent pandas) and some managers telling the devs to just frigging do it regardless.)