What a good question!
I don't know if I *need* anything as old as 10 years, but I nevertheless keep backups of all old accounts. My preferred tool these days is "offlineimap" which makes it easy to keep email on file.
This probably isn't for everyone but I purge everything older than 7 years, and I've stopped using labels altogether. It's either in my inbox, or archived and I search for it. Actually I do still have one label, for @idpro_org.