Calling it unusable seems like unnecessary hyperbole. That said, it does seem like a step backwards! Do these things get A/B'd with static signup pages? Surely they are going to lose some signups for this. Maybe they don't care!
Come on, it's 2023. It's more than reasonable to expect all your users to have a login-form accelerator in their PC. If you're not keeping up to date, the fault is with you, not the web developer. /s