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.
I don't mean to be argumentative, but not all BIPs are proposed and "accepted". There are BIPs, such as BIP 135, that are proposed and rejected. If you read BIP 39, you will see that its status is
proposed.
Anyway, there are no "accepted" BIPs, unless by "accepted" you mean inclusion in Bitcoin Core.