Is it ethical to explicitly or implicitly bias against student papers unless co-authored with senior researcher(s)? A few days ago I was browsing open-access social-science journals and I found one ‘author guideline’ explicitly saying that they are accepting student submissions if and only if the manuscript is co-authored with the academic advisor or a senior researcher.

I have acquaintances with several editors and editorial board members at various journals and I know from experience that some of us tends to be careful with manuscripts coming from PhD students. This manifests in practices that I find questionable (e.g., insisting on three unanimously positive specialist peer evaluations before acceptance, while a senior researcher’s manuscript may go with only two positives) and in other practices that I find to be biased (like an easier desk reject, or being less willing to deal with them, which results in longer peer evaluation cycles).

Is it justifiable for editors to restrict – formally or informally – the opportunities for PhD students to publish in their journals, like restricting them to submit only co-authored papers or subject their manuscript to more strict criteria than those of senior researchers before acceptance?
In my opinion, such a policy is absolutely unethical.

One of the key principles of science is that scientific work can be done by anybody.  Good science can be done by people who aren't even in academia, let alone by students.  A scientific paper should be evaluated on its merits, not an argument from authority based on its authors.

Now, humans being humans, reviewers will tend to bias towards known authorities and prejudice against unknowns, students, etc.  But this journal's policy, rather than fighting against such a tendency, explicitly enshrines and adopts it.  Likewise with the informal policies that you describe: anything that amplifies unfairness in judgement rather than de-amplifying it is highly suspect and likely scientifically unethical.

This doesn't mean that one should drop standards.  Rather, it means that one should treat a paper coming from a well-known PI at an august institution with just as much suspicion as one from a student author from a developing-world university you've never heard of before.  Being status-conscious primates, we're not very good at doing this, but such a condition is the goal towards which we should strive.

PS: Note that a policy the other way, i.e., student journals that don't accept papers without student authors, is not problematic because it is inverting the privilege gradient.