We use 12-words seed phrase to backup up nsec
(and, to be precise, it backs up the seed, out of which the nsec is generated. It's not 'generate seed as nsec backup' but more 'generate nsec out of seed')
We don't know what app has used 8-word derivation, it doesn't sound as best idea...
it's pretty silly, indeed
i tried to propose a 24 word EXACT nsec backup word key that uses BIP-39 dictionary... it only had 7 bits of checksum in it tho, so a small chance of failing to write it down correctly, but highly unlikely
the 12 word key doesn't let you generate all possible keys... this is why the Brave Sync key is 25 words, this gives you full 256 bits of exact key plus 18 bits of checksum to make sure you don't mess it up
