Keeping a portable long-term offline wallet I'd like to keep some bitcoins as a long-term investment - store them to a pen drive which I stash in some safe place, or otherwise backup it securely, then remove all traces from my online computer never to worry about this particular wallet getting stolen except by an old-fashioned burglar.

Now my worry is that if I export my wallet with the coins from client program X, which will pack the wallet, the keys, some configuration and other stuff into its own proprietary backup format, then the author ends the project in a year, in five years the OS changes and no longer supports this kind of binaries and their API, and when I finally decide to cash my investment I find myself with a data file no bitcoin client on the market can process.

What is the most portable to survive for years (probably most "raw") bitcoin wallet format? Which software can export/import it currently?
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.