Account sharing is still punishable and is a violation of the Extended Tibia Service Agreement, however not a Tibia Rule! The punishment depends on a case by case basis. I will paste some important responses from Cipsoft to back up my answer:
"Q: What about account trading/sharing statements? A: If we receive statements of this kind, customer support will process them. We expect players not to share or trade their accounts, and we strongly advise against doing so as we know from experience that these practices can lead to all kinds of problems for the players. However, since there will be no dedicated Tibia Rules against the sharing and trading of accounts, different criteria will apply in regards to enforcement and possible sanctions, and such cases will be handled on a case by case basis.
Players who share or trade their accounts and get into trouble as a result can expect no support from our side. Moreover, we reserve the right to apply further sanctions based on the individual situation; however, there are no standard rules of procedure. Please note, though, that violations of the Service Agreements are the sole concern between the contractual partners, i.e. us and the holder of the account. For this reason we cannot provide any further information in regards to the account or possible sanctions." Source: Character Auction FAQ
Here's what news about Character Auctions had said regarding the Tibia Rules changing: "An account includes personal information about its holder, e.g. payment information and registration data, and is a key element of the relationship between a specific individual (the account holder) and CipSoft. As such, an account is untransferable and we do not condone the trading or selling of accounts. It goes against the terms on which CipSoft offers you access to an account. Therefore, we are going to add a new passage to the Extended Service Agreement, for cases in which the agreed upon stipulations have not been upheld. In return, Tibia Rule 5a) Account Trading or Sharing will be abolished. The Service Agreements will be amended on September 14, when every player has to agree to the new terms and conditions. Please note that the new terms of service will only be available from this date forward, not before." Source: News: Character Auctions
Here is what the Extended Service Agreement currently says: "5. Duties of the Users: All holders of Tibia accounts are responsible for the security of their accounts, registered email addresses and computer systems. They must not disclose account data to others or accept account data of others (including account trading and account sharing). CipSoft GmbH cannot be held responsible for any damage caused by compromised accounts." Source: Extended Tibia Service Agreement