Is silence on a consent request implied non consent?

No, it is not. You don't have consent after your implied consent runs out, but ignoring your request for consent does not count as opting out of future contact. In addition, if they renew their business relationship with you (inquiry or purchase) they renew their implied consent, even if they had previously said they did not wish to join your mailing list. Customer care best practices may be to keep a list of persons who never want to receive communications from you, to avoid friction with customers that just don't want emails.