I got hit by that, basically forced me to make a Google account and add all my sites to it even though I couldn't care less about SEO and indexing. Now it keeps sending me spam emails about "problems" with my websites. No, I'm intentionally not letting you index this.

What seems to be going on is it's flagging random widespread open-source software as impersonation/phishing login page because it's seen it on a bigger site and assumes you're doing some phishing.

Filed an appeal and it thankfully promptly got resolved. Google ain't known to be friendly to developers.

I want to like that feature because I'm sure it's helpful for the less technically savvy. But I hate that Google can just decide my site is unsafe and essentially cut my sites off the Internet for most people. If Google denies your appeal you have basically zero recourse.