Does macOS still use Yarrow as it's cryptographically secure pseudorandom number generator?
Yarrow is gone. 

The kernel CPRNG is a Fortuna-derived design targeting a 256-bit security level.

https://support.apple.com/en-ie/guide/security/seca0c73a75b/1/web/1

Ok! So I mailed Craig Federighi recently about this, concerned that macOS wasn’t capable of generating >128bit quality keys. 

He responded!


  ‘The source you were referencing is out of date.  You can find more current informations here:   FIPS certification document, section 7: "The NDRNG feeds entropy from the pool into the DRBG on demand.  The NDRNG provides 256-bits of entropy.”’


What a legend. 



So yup, macOS and iOS are both capable of generating 256 bit entropy and creating quality CS keys for both ECC and Symmetric Key crypto. 

EDIT: Apple confirmed the kernel CPRNG is a Fortuna-derived design targeting a 256-bit security level.