Is it permissible to agree that the increase in apartment rent be by mutual consent between the tenant and the landlord, contrary to the customary and common practice of a 10% annual increase?

There is no objection to the aforementioned contract if the increase occurred after the expiry of the agreed-upon period by mutual consent. For the determining factor is what the two contracting parties agreed upon, not what the labor law stipulates, as the labor law is applied unless the contract states otherwise.