Serfs were bound to the land and not much better off than slaves. Another lord would not have welcomed a runaway serf and escaping to a city would have been an extremely risky venture as serfs had little – if any – money (subsistence farming was the order of the day, when not working for the lord on his lands). Giving up their family and wider social support network to become an anonymous beggar in a city would not have been a good move. Even if one had a skill, the city guilds controlled the ability to use skills for commercial purposes. It’s possible that a few people (e.g., gifted musicians) may have been able to escape the bonds of serfdom. I also don’t know how easy it was for a serf to join a monastery or a convent – but otherwise, I imagine the proportion of serfs who could run away to the city is likely to have been equivalent to the proportion of modern-day Mancunian kids who escape the council estate by playing for Manchester United.
"Stadtluft macht frei" and the related "year and a day" are still German proverbs. Eric Hansen made similar assertions here (popular history, but sounds plausible): https://www.amazon.com/-/dp/3596173248/
I knew the experience of European serfs varied a lot in terms of freedoms and obligations to their lord but everything I’d read until now about life in feudal Europe lead me to understand there was very little mobility – and particularly so for the lower classes. That Wikipedia article provides an interesting counterpoint so thanks for that.
I’d also only come across “a year and a day” as the pre-marriage trial period that a couple would spend together in the tradition of hand-fasting – and the old common law standard regarding attribution of cause of death. I was not aware that it also served as the length of time that escaped serfs would be legally freed if they managed to survive that long.
