Does Charcoal Powder Have an Expiry?

The short answer is 'No'.
Providing that the charcoal powder is not exposed to moisture or the environment in any way where it will adsorb toxins and pollutions, it does not have an expiration date.
According to Dr. Agatha Thrash and Dr. Calvin Thrash in their book 'Rx Charcoal', 
"Charcoal powder has an indefinate shelf life, remaining effective for years, probably decades."

*We advise checking with your government body as some countries have regulations for expiry dates. 

References
1. Charcoal and Body, Frequently Asked Questions, https://www.charcoalandbody.com/frequently-asked-questions/

2. Thrash, Agatha and Calvin, 1988, Rx Charcoal, Sunfield MI, USA, Family Health Publications LLC