Not in the U.S. AFAIK. Every employer-sponsored medical plan I have seen is optional. Each employee decides whether they want the plan (often with a choice of plans) and the payroll deductions that go with it.So why is the employer's health/dental plan such a significant issue for jobseekers? Fact is that many people *need* the benefits of an employer's socialized scheme because of pre-existing conditions, cost of singleton policies, lack of negotiating muscle if it's you vs the insurer... socializing is the typical response to protect against the powerful and their expediencies.
You are correct that this is voluntary, just as wearing a coat in Chicago in December is voluntary. ;-)
"... They ne'er cared for us
yet: suffer us to famish, and their store-houses
crammed with grain; make edicts for usury, to
support usurers; repeal daily any wholesome act
established against the rich, and provide more
piercing statutes daily, to chain up and restrain
the poor. If the wars eat us not up, they will; and
there's all the love they bear us."
-- Shakespeare: Coriolanus, Act 1, scene 1
