Keeping a portable long-term offline wallet
All wallets basically just store public+private keys. If you store those keys, it's fine.

Also, there's the normal wallet file, which is stored in the Berkeley DB format, which is an extremely common format and it has been for the past 15+ years. While I can't speak with 100% certainty, I'd say that the wallet.dat file from the reference client is a pretty safe bet (perhaps even safer than storing the raw keys).

Which software can export/import it? Well, there may be a few exceptions, but almost all Bitcoin wallets, either online or offline, can handle those files.