I remember the same, but funnily enough, it seems to be fixed now, with both "Sign in" and "Sign up" buttons available on the top right. I don't remember how it was before, but I always started to create a new account instead of logging in.

It's significantly worse if your browser is half the width of a 1080p screen, which mine generally is.

Yep, even worse on mobile. You have to open the hamburger menu and then scroll 4+ screen heights down (on a OnePlus 8 Pro) just to find "Login".

GitHub is one that gets me often.

What's the problem? They have both "Sign in" and "Sign up" next to each other. Same color, opacity and the only difference being that there's a border around "Sign up".

On mobile, the sign in link is hidden behind a hamburger menu.

Yes, that's what gets me.