Pre-Industrial Societies: Anatomy of the Pre-Modern World ( https://www.amazon.com/dp/1780747411/ ) presents serfdom as one outcome of a battle between the head of state and local nobility that was common across societies, with serfdom being what the nobility wants and freeholding being what the head of state wants. The analysis is that each side wants peasants to have the status that makes tax collection easier for their side.

Interestingly, those positions have reversed in the modern day, with the analogue of serfdom -- W2 employment -- heavily favored by the state, and the analogue of freeholding -- employment as a contractor -- preferable to employers (who mostly lack the power to implement it over the state's objection).

Your modern day analogy fails.

Corporations prefer contractors because they can use the power asymmetry to extract more value for lower pay.

Formal employment is favoured by a democratically elected state, as it works to restore the power balance to the voters through appropriate regulation (assuming of course you live in a modern well functioning democratic society - which I understand is not the case in many places).

I strongly disagree. Having been both an employee and a contractor, both in the gig economy sense and the consultant sense of the word, I very much prefer and felt much freer as a contractor. I think the reason states prefer people to be employees is probably the same reason the Roman state mandated farmers and sharecroppers stay on the land, it makes us much easier to tax and control.

There is probably also the added benefit that it's more difficult for employees to move into a position were they can compete with the corporations that pay for the politicians campaigns in democratically elected states.

There are real benefits to the usual employment arrangement that have nothing to do with power asymmetry or making it easier for the government to turn people into serfs. The employee might benefit from that arrangement because it lets her insure against inevitable random fluctuations in productivity, and stabilize her income. (Newsflash, "profit sharing" is a bad thing for most employees. It's not what they generally want.) OTOH, the employer can lower workers' turnover or 'churn', and incent them into contributing to long-term enhancement of firm-specific capital. Building this sort of capital is in fact one of the main ways in which workers can contribute to any organizational endeavor; even though it generally happens tacitly, it can be far more relevant in practice than any overtly assigned 'tasks'.

There wasn’t one thing we can trace serfdom to. The system we usually envision is a product of Charles Martel but before him there were serfs all the way back into the Roman Empire. It was more a natural evolution/long term trend of the Roman economy away from “slave based” labor.

It is indeed often cited as the start of the medieval peasantry.