> Such understandings are very common across competitors in all lines of work, whether they're written or not; at a former company, I was personally told by the CEO that we couldn't actively recruit someone who worked at a competitor because we didn't want to risk starting a bidding war over talent and potentially throwing everything off-kilter.

I think this is one of those occasions where it's OK for companies to individually come to this conclusion and not implement this as a practice. But the moment several companies come to a collective agreement on the same, it enters questionable and probably illegal territory.