Template stacks were a large part of Stacks 3, but have been deemphasized in Stacks 3. They still work just as well, and Stacks ships with the same built-in ones, so your sites that use those templates can still use them, but they don't appear in the Library by default.
see this answer for how to display the template stacks in your Stacks Library.
Why were they deemphasized?
There are many reasons, but it mostly has to do with Partials. Partials more elegantly solve many problems that Templates, User Stacks, and Globals all tried to solve. Partials are easier to use, faster to publish, and much more powerful. Deemphasizing templates helps focus people on doing things with Partials -- which are just a lot better in every way.