I’m no expert, we did this with support from another company more familiar with IDPs, but if you use an IDP you can make your login page look however you want. We’re currently using Google and Microsoft accounts linked to the IDP configured in Ignition, and we use Ignition security levels and rules to admit domains and specific user accounts. So yes, you can have a custom login page in this way.
However, in some cases when the system logs out by itself due to timeout, it redirects to the normal Ignition login page. If you try to login from there it redirects users again to the main IDP login page configured on the perspective project.
In terms of actually “changing” the default Ignition login page with some sort of customization, yes, it is unreachable.
A year or so ago I tried making a perspective project that functioned as a “project gateway” and I made a custom login project. The project was public, so it skipped the login page, but I encountered too many issues that I couldn’t handle via scripting properly. So I gave up on it. There’s been many updates since then, so maybe you could try this. But if you want to avoid the ignition login view completely I’m not sure it’s possible.