The phrase "subject to renewal" is correct and usable in written English. You can use it when something is conditional on a renewal or extension, usually referring to a contract or agreement. For example, "My employment is subject to renewal every six months."

the act of making an agreement continue for an extra period of time after it has come to an end: contract / licence renewals The purchases are subject to annual renewal.

It typically means that something is renewed or repeated annually or on a yearly basis. For example, you could use it in a sentence such as: "This contract is subject to renewal every year.".