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.
While BIP 39 is a great feature, SLIP 39 is a major improvement. I hope that enough wallets eventually adopt SLIP 39 so that it becomes as ubiquitous as BIP 39, perhaps even replacing it.
Although SLIP39 is an innovation from Trezor that aims to solve the problem of lack of trust in delegating recovery actions to a certain number of people or places and determining a minimum threshold to rebuild the wallet, I don't think it should be replaced, BIP39 is still very useful and the most trusted by people and wallets, what we want to see here is an additional option and not to exclude one that already exists, it's worth remembering that SLIP39 does not make the BIP39 standard obsolete.
I personally will continue using the BIP39 standard, I do not see a specific reason to use SLIP39 other than for testing for now.
Remembering that in BIP39 it's possible to use the BIP85 standard to derive child seeds, which can be very useful if we want to use only one seed as a backup and this will be used to recover the child seeds.