I think employees have been allowed to quit at any time since the early 20th century in most western countries. Of course there is a probationary period involved, but it's nothing like earlier times when police would possibly come and take you to your place of work, or it was illegal to be unemployed in several jurisdictions up to 20th century (e.g. USSR) etc.
So discussing this as "feature" rather than tablestakes in any modern society is somewhat misleading IMO.
I don't know if there are some professions where it's a bit more complicated than this and am happy to be corrected.