Is it permissible for a company to compel employees to waive their right to annual leave without deferring it or compensating for it, and is this considered one of the terms of the contract or is it an injustice to the workers?

The company is not entitled to compel an employee to waive their rights without their consent. It must treat them fairly and restore their rights to them, unless they willingly waive them, based on the Almighty’s saying: "O you who have believed, fulfill [all] contracts." The worker's right is what was explicitly agreed upon or what is customarily practiced and not contradicted in the contract. If the employer does not fulfill their right, the worker is entitled to demand it and file a lawsuit against the employer with the competent authorities.