You’ve taken it away in 3.5 and there’s no option not to use full screen login. Full screen login and sign up when enabled previously had a massive negative impact on our signups so we had to revert to modal and now it’s doing it again since updating to 3.5.
Why have you taken away the option for us to choose?
Hello and welcome back to our community! 
Can you tell us some more about the impact this change has had on your site, and how you know it has had a massive negative impact on signups? Could the decline be related to something else?
I ask because I have not heard others complaining about this change, which brings Discourse in line with industry standards and has otherwise been very positively received since we introduced it last year.