It sounds like your system tracks the expiration so you can begin withholdings if you don’t have a current W-8BEN on file. If you do not have Form W-8BEN in your files, you are required to withhold taxes on interest paid.
I don’t believe there is any required language for a notice to your customer. I recommend you consult with your accountants or tax advisors to for language to explain the IRS requirements and consequences of not having a current W-8 on file.