From an announcement on Discord:
SSSL doesn’t work because BOSS and SpotPass issues. There is no estimated timeline on a fix. As of now the only known solution (that I have heard of) is to HomeBrew your console and use Pretendo that way. I am sorry for any inconvenience this may cause.
Condolences,
Luni Moon
There are two threads about SSSL and Splatoon, both with statements by Jon:
We are aware of issues regarding SSSL and Splatoon. These are being looked into.
You should add that this is currently temporary. […] We’re looking into why this happened as it SHOULD be supported
Will update if the situation changes.