Do ordinary employees of the State Administration of Taxation need to apply for an entry-exit certificate letter in their original unit when they retire for 16 years and travel abroad?

generally speaking, it is unnecessary;

according to the relevant provisions of China's civil servant law, public servants who work in the state secrecy industry must go through a certain period of declassification after leaving, retiring or dismissing. The length of this declassification period is different according to the confidentiality of the content you are exposed to. If it is general confidential information, it may be enough for one or two years. However, if it is tax data with a high degree of confidentiality in China, the declassification period may be very long, but in a nutshell, it will be 16 years.