You can remove username enumeration from login and password reset, but it's much harder to remove it from signup. And this is pretty much an all or nothing, there's no point giving cryptic error messages (in the case of forgot password) to users when hackers will just use the signup form anyways. Want a good example? Hacker News: "That username is taken. Please choose another." I just tried on github, generic error message on login, explicit "email in use" error on signup. Amazon? Same thing.

I'd wager that 99% of website allow username enumeration. The very few cases that don't, typically shield their signup with additional steps (captchas, mobile OTP (e.g. paypal, ...)). None of which are foolproof and all of which are a compromise not just from a security point of view but also from a usability point of view.

Until someone gives me a great way to do this with signups, I'm going to stick with the much more usable forgot password error message.