I don't know about TLS Sign, but if I had to venture a guess, it was likely dropped because PKI provide a certain level of Non-repudiation through CA's. TLS is an encryption protocol, it's primary purpose is encryption. Non-repudiation is generally considered a separate service in those regards.
Digital Certificates are what we typically use for standard non-repudiation, it provides a common platform for two unknown parties to trust that each other are legitimate connections.
A company for instance, must register its Certificate with a CA, companies than go through some sort of validation to establish the legitimacy to the company saying who they are. Once a certificate is issued a user can verify a presented certificate, with the trusted CA's, and know that the connection they are establishing is a secured connection with a company that has passed their screening.
Since this can get long, I know there are concerns with how good PKI actually is, and security around certificate authorities.
Non-repudiation is nice, but TLS isn't focused on that. We use the PKI with TLS to provide both encryption and Non-repudiation, if you are looking for a higher degree of non-repudiation (such as ensuring a file is send by a certain user or company) TLS would not be the right place in my mind.
-Edit Typos and clarifications