Ask for advice: how to inform customers when employees leave?

You can inform them one by one through letters with company seals, e-mails, text messages, telephone calls, etc.

Dear customer: Mr. Chen, the former post employee in charge of XX business in our company, officially terminated the labor relationship with our company on X, X, and from now on, the XX business in charge of Mr. Chen was changed to Mr. Li (contact information: ……), and all the comments made by Mr. Chen will no longer represent our company. Please pay attention. Please forgive me for the inconvenience. XX company x year x month x day

If it's a core customer, it's best to let the new business personnel call in person to tell the situation, and also have a preliminary understanding and communication with the customer by the way.