InMobi has some data showing that SKAN 4.0 readiness is (unfortunately) still very low. And even that number doesn't tell the whole story, because 'compatibility' and 'being used' are not the same thing.
I've said it before, but here's your reminder: because there is no required timeline from Apple, and the previous versions of SKAN will continue to function just fine for the foreseeable future, we're looking at an extended ecosystem transition period. The lingering specter of rampant fingerprinting abuses (on which Apple has still not yet cracked down) further erodes the incentive to update quickly. And that's unfortunate, because SKAN 4 is finally approaching 'usable', where previous versions were just painful.