"Voluntarily" is doing a lot of work there. There is a huge power imbalance between employers and unemployed people looking for a job, especially as they may face bankruptcy and homelessness if they go too long without income.

Still less of a power imbalance than talking about the government. Let's not forget that the government is the one who enforces those non-competes. So you're really only talking about one subset of the government authority.

FWIW, and not meant as a contradiction: while (talking Britain here particularly) many were still literally slaves under Feudalism (not serfs) and others Villiens (serfs tied to a property - the root of the once-classist word villain); there were also a great many free people taking contracts under bad terms, if only because it's easy for Lords to collude re wages for servants, etc, etc.