But there’s a site setting for that.
Search for default topics automatic unpin and automatically unpin topics . When a user read it, the topic remains pinned
EDIT: if you disable only default topics automatic unpin , users can change this setting in their preferences/interface. I think this choise is the best for your users.
If you disable both settings, topics remain pinned forever (users cannot control it at all, and for regulars it can be annoying).