I'm pretty sure that executives have legal liabilities that non-execs lack, although that might apply in most cases just to the top execs.

Particular roles (CEO, board member, CFO, ...) have particular legal responsibilities. I don't think there's any law addressing executives or "top executives" as a class.

There are lots of duties in the army that only an officer can legally do but an NCO cannot, but not the other way around.

There are roles that require being an officer, sure. But more technically, they always require being an officer of a certain minimum grade. Other positions require being a non-officer of a certain grade or above. And plenty of technical positions require specific training and certification, regardless of officer status.

I'm thinking of very large companies, including those I have worked in; and those I have read about. Startups allow agentiness to everyone.

I would describe it as startups requiring agentiness from almost everyone qualified to work in a startup. Otherwise it becomes just a small company and probably fails.

The startup where I work is just now transitioning into a post-investment non-startup company with a board of managers. And so, for the first time, we're explicitly looking to hire non-agenty people to fill some junior roles. Agentiness is very much the word I'd use to describe some of our staffing decisions. But we still don't have anything like an exec vs. everyone else distinction. I'm one of two technical architects, a programmer, very agenty, and definitely not an "executive" (and I don't have any legally binding duties beyond an ordinary employee with a contract).