U.S. companies have frozen recruitment, are the offers already issued still valid?

Effective.

A US company’s recruitment freeze means they are no longer hiring, while an offer that has been issued means they have already admitted you.

Offer, the full name is offerletter, which is interpreted in Chinese as acceptance letter and admission notice. There is no unified name yet. Some people call it "admission notice", some call it "acceptance letter", and some people call it " Letter of Invitation". An offer letter is used to formally offer a position to a job seeker and provides important information including start date, salary, hours worked, and position.