A question that came up considering new ciphers/macs/compression/etc.:

How long shall attic support backups created by older attic versions?

Forever? That would either mean code gets more and more complex OR that development is blocked from supporting new stuff.

I see the need to be able to read old backups, but "forever" seems to be too much / too harmful.

In any case, defining this policy would be useful, currently it is (AFAIK) undefined.