While BIP 39 is very popular, it is not the "official bitcoin standard". Strictly speaking, there are no "official standards" because there is no authority; however, a BIP implemented in the reference client might be considered to be an "official standard" for practical reasons
FYI, Bitcoin Core does not implement or use BIP 39.
Bitcoin Core is using master private key instead. BIP means was proposed and accepted. If the proposal is not accepted, it will not be included into the BIP lists just as SLIP39. BIP39 was accepted which makes it official.
SLIP39 which is shamir's secret sharing for mnemonic codes is the one that is not official. Also I prefer seed phrase which mist wallets supports. I will prefer to add passphrase. If it involves more than one person, I will prefer multisig wallet.