I don't know if this exists in the form of an extension, or if Flarum has considered adding this as part of the core. I believe this should be included in future versions, although I understand it's something complex if it wasn't part of the initial project base.
Many forums use a discussion archiving system. This is especially useful in large communities. Closed or archived discussions are placed under an "Archived" tag and are usually moved to another table. This way, the bulk of queries to the "discussions" table becomes more efficient as its size is reduced. In the end, this is the table most frequently queried by Flarum, so as it grows, it generates more issues, no matter how efficient the queries are.
Archived discussions can always be accessed by users through this tag, but instead of querying the "discussions" table, it can use an alternative "Archived" table. In general, these discussions would be low-traffic, obsolete, or irrelevant to the forum.
