I was just about to ask that question: what is the basis for the prohibition on selling shares? How are those prohibitions enforced? If the basis is contract law, then you can't necessarily prevent people from undertaking the prohibited action, can you? Just collect damages if they do.

If an employee sells their shares in violation of a company contract, what happens? Can the company repossess the shares? Or are the shares structured so that they can be made worthless?

Can employees construct a security based on their shares and then sell that instead? Nominally the employee retains the shares, and sells a contract based on them to someone else - an asset-backed security. The contract is an option that allows the contract-owner to request the shares-owner sell their shares in the event of liquidity, and the contract owner will receive the gains. Alternatively, the share-owner simply holds onto the shares until they are transferrable (eg IPO). In exchange for this the shares-owner is paid an up front fee.

Presumably this kind of arrangement could also be prohibited in some way by the company, but if someone is willing to violate that agreement, it would be difficult to discover that the agreement exists.