What is property?

Please cite the passage in which Rothbard talks about such clauses.

Argument by terminology? Argument by tradition?

You claim to have read this stuff, but your comments say otherwise. From the second response in that thread:

Hence, the unenforceability, in libertarian theory, of voluntary slave contracts. Suppose that Smith makes the following agreement with the Jones Corporation: Smith, for the rest of his life, will obey all orders, under whatever conditions, that the Jones Corporation wishes to lay down. Now, in libertarian theory there is nothing to prevent Smith from making this agreement, and from serving the Jones Corporation and from obeying the latter’s orders indefinitely. The problem comes when, at some later date, Smith changes his mind and decides to leave. Shall he be held to his former voluntary promise?

The point is the enforcement of such contracts is not in line with libertarian ethics.

Proof you either haven’t read what you claim to, or simply don’t understand it.