Retention time - provide posts indefinitely

In general I like the idea of posts disappearing after a certain time. Now especially e.g. on reddit you often though have an exchange about something that is worth being discoverable even years after the first post. For instance let’s say you are discussing which free and open source alternatives exist for a certain application. How can you provide that information longer than 6 months?

Would be nice if the mods of a community could define one or multiple threads as important and pin them to the top without having them expire. Maybe put a maximum amount of pinned threads per community, between 2 to 4?