I don't mind a no-cold-call/no-active-solicitation agreement between companies. (That is, even if it is illegal, I don't think it should be illegal.)

I especially don't mind it if, as the great-grandparent suggests, the existence of other non-cartel competitors meant it had no effect on salaries.

Even if it did have indirect effects on salary levels, such conventions might also plausibly help prevent certain kinds of ruinous/negative-sum-for-all competition, that rewards a few rapid job-switchers, but at the expense of the industry, region, or majority of employees.

I would mind a "don't-hire-if-they-apply" agreement. That'd be an abusive oligopsony conspiracy in restraint of employees' ability to seek out new arrangements.