Thanks @Monte_Kalisch @nathank @downey! Amazing to know that this idea didn’t exist only until community members shared it, thanks so much! And thanks to the discourse team for supporting it. I hope this specific feature is not rolled back 
with the new modifications to the invite system.
@tobiaseigen, this is exactly what we are facing now with the invite links page which doesn’t have the login button at the top header for existing users. It’s confusing existing users to see a form to ‘sign up’ rather than an option ‘if you already have an account please login’ 
(No 'sign in' option in the accept invitation page, only the signup form is shown - #12 by gassim)
Yes, a fix is on its way I think to add a tiny ‘login’ text at the bottom of the page, but it will not be clear for existing users (especially if they are “new” in the first place (i.e. already registered but unfamiliar with the platform) or if they are not very tech savvy.) The expected reaction is that existing users will think the invite link is only for ‘new user’ to sign up, or they will click the homepage logo instead of the tiny ‘login’ text at the end of the page. The issue is that if they don’t see the tiny login option at the end of the ‘sign up’ form they will not be redirected to the intended topic.
I think hiding the login button is not going to be helpful for existing users. I also don’t think that new users will consider the ‘login’ button at the top of the page instead of a ‘sign up’ form.