I think when you log in, there should be no requirement to select "username" instead of "email" for users who log in with a username
the reason is simple, no username has an @ symbol in it. so, programmatically you could determine if the input is an email or a username based on the presence of the @ symbol.
this is done on almost every major website. for example on gmail, you can log in with a username or an email address. i.e. if your address is [email protected], you can simply login with username. or you can log in with [email protected].
this is much more user friendly, especially since the form doesn't remember your last setting so you have to do it every time if you do not use email. honestly I am surprised no one on the website team had this idea or allowed this to get deployed because it's incredibly inconvenient to older players.
there is no reason to require us to select username. I even have a browser addon I am using to circumvent this weird issue. but I use my phone a lot where I cannot use the browser addon and should not have to deal with this problem.
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Eventually, we'll all be using email logins when converting to the Jagex Account system becomes required to log in. So, this is extra step is already pretty temporary.