[lusr]

A clause restraining you from engaging in your "profession, trade, or business", i.e. in general, is unenforceable. A clause restraining you from engaging with another party may or may not be unenforceable depending on the details.

In other words, nobody can enforce a clause preventing a programmer from programming for company B after leaving company A purely on the basis of restricting competition.

They may have a case if e.g. company B contracted company A to do some work and then bought out company A's programmer (e.g. to save money), but only if the contract between company A and the programmer in question has a restraint specifically penalizing this scenario.

(Even then, it doesn't mean companies will choose to enforce the contract. I've worked for companies where the restraint was between the companies A and B, and in the interest of preserving business relationships, company A decided not to pursue their legal right to recourse when company B poached an employee from A; the employee did not break conditions of their employment with A as the restraint was between their companies.)