I'm about to wipe my MacBook Air and am wondering if I need to back up my local archives of app versions that I've already uploaded (including symbols) to the App Store. When I install Xcode (on a clean install of macOS) and log in with my developer account, will Xcode automatically download and show these archives and their crashes?
You do need to manually backup your local archives. These are not stored in a cloud for you to pull down on a new system. Crash logs and dSYM files do come from Apple's servers, however.