Requiring a formal research plan can still serve as a filter for researchers who are not serious or disciplined, even if they don't stick to it exactly.
I personally don't agree. Anyone can make a Gantt chart, it's basically busywork. If it's required, every applicant will include it, serious or not. In fact is the kind of thing you could even outsource to a consultant (if you had access to one) and now, of course, to an LLM. Where seriousness matters is in actual execution. Which takes me back to what I previously said: we would probably be better off evaluating just the CV (which has signal about past execution) than evaluating project proposals.