If you want to keep the historical messages in an inbox, but not use it anymore, we recommend simply removing the channel from the inbox. 

For an explanation of the difference between inboxes and channels, you can see here. We also like to describe it like this: The inbox is your mailbox, and your channel is your mailman. The inbox is where the letters live, but the mailman is what actually brings those letters in.

If you fire the mailman, no new emails will come into that inbox, but all the old letters will be there.

We recommend changing the name of the inbox to something like "Inbox: deprecated" just for good measure. That way, the team will know it's no longer in use. If the inbox is pinned to teammates’ sidebars, they can unpin it by editing their sidebar and unchecking the inbox.