A subdomain like abtest.questionai.com is typically used by developers to run split tests or experiments. It's not a store, a login portal, or a service you're supposed to interact with directly — which is why it currently shows a 404 error.
For this kind of subdomain, many of the usual trust signals (contact info, about page, privacy policy) simply don't apply. Most established companies run test domains that are invisible to the public. The fact that it has a valid SSL certificate, a clean security record, and no blacklist history is consistent with a legitimate internal tool.
The main thing to ask yourself is: why were you sent here? If you clicked a link from a trusted source (like a reputable company's email or a known website), it's likely part of their normal testing workflow. If you stumbled upon it randomly or via a suspicious link, there's no evidence of harm, but there's also no reason to trust it with any personal information — because there's nothing functional to interact with anyway.