Ever since I've been responsible for user-facing applications -- which is probably since the early Jurassic period -- and ever since I've been using pentesters on those apps, which was probably two seconds after the Jurassic was over -- I've run into the same problem, over and over again.
It's the ridiculous security trope that "username and password feedback is bad."
It's one of the first things that a pentester points out: you can find out valid email addresses or usernames by putting in a bad one and looking at the response. Yes, I know this to be the case.
IT'S ON PURPOSE.
Anyone who has had to provide user support on an application knows how much of that burden is due to users forgetting their usernames, or forgetting which email address they used. Remember: you have one application. The user may have dozens or hundreds of accounts in applications across the Internet, some of which they may use only once in a few years. It's unrealistic to expect them to have been writing down which usernames, email addresses and passwords they've been using since the '90s (especially if they were assigned those usernames - remember when that was a fad?). Unless you think it's okay for them to have saved everything in their browser ... no? I didn't think so.
It's bad enough when they get a clear message saying "We've never heard of you," and they're sure that they do have an account on that system.
Can you imagine how much worse the support load gets, and how much more frustrating it is for the user, if the application refuses to tell them what's wrong?
We may or may not have sent a password reset email to the address you typed in. Even if we sent one, you may not be registered on our system, so the password won't do you any good. Ha ha ha. Take THAT, HaXX0rz!!!1
If you want to prevent user enumeration attacks, you had better have a good alternative in mind. You CANNOT forget the real reason the system is there, and what that user understands and needs in order to use it. If you can't suggest anything that helps, then you are myopic in the extreme, and you're probably just playing reindeer games with other hackers, not contributing usefully to the business that hired you.
Treating username feedback as evil is just playing into the "security by obscurity" mindset. If your system can't withstand attacks by someone who knows a valid username or email address, then you have MUCH bigger problems to solve. Throwing your users under the bus because it's easier for YOU is not the way to solve them.
Thank you for listening.