It was _13_ words long by default when I created a new wallet. (linux version)
Interesting, I've not seen it do that with newer versions of Electrum.
Thanks. So, I can create a BIP39 seed mnemonic in any other wallet, restore it in Electrum and be sure that I can restore all funds with this seed in the future in any BIP39 compatible wallet, right?
Theoretically, yes.
Be stuck with an irreversible 13-word seed in case Electrum ceases to exist some day is a more serious thing.
The algorithm for converting an Electrum seed mnemonic -> seed -> addresses is open source and published... it can't just magically vanish. Even if the developer stopped working on it, then are plenty of copies of the code around... and it won't make your currently installed version of Electrum magically stop working either.
It is a little frustrating that Electrum uses non-BIP39 compliant seed mnemonics by default (I do understand
why ThomasV has chosen to do so), but it is good that it supports using BIP39 seed mnemonics if you create one elsewhere. Just make sure you use a good "offline" wallet to generate the seed mnemonic and also that you understand how to minimise the risks of leaking that seed mnemonic during creation and/or entry into Electrum. Also, make sure you have solid offline backups, because like I said, you can't recover it from the wallet file generated with a BIP39 seed mnemonic (although, the wallet file will still contain the xprv) like you can with a wallet file created by a "native" Electrum seed mnemnonic