Well, users can unpin anything we pin if they wish to, at the bottom of a topic. There is also a setting that is (sadly) enabled by default, which unpins a topic when you get to the bottom.
This setting can be found in your settings > preferences > Interface
Keeping topics pinned at all times would be a global backend setting we have no access to. There is no possibility to have some to always stick, and others to follow default, for as far as I am aware.