Volume names in deployments have to follow DNS label standard as defined in RFC 1123. That means:

• contain at most 63 characters
• contain only lowercase alphanumeric characters or ‘-’
• start with an alphanumeric character
• end with an alphanumeric character

However, right now we use in our deployments (at least in the Kafka Connect one the resource name) - for example in authentication secrets. (try with a user with .)

For the record, the volume name restrictions are different from label restrictions.