They're both practically interchangeable. What's the rationale for this change?

The rational is that we are using these generators downstream, and providing the argument, when we want to construct say a validity interval, seems like unnecessary noise when it can be inferred from the era type.

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Having an era just on a type level seems like an inconvenience when working with multiple eras. See this test for example:

How would you rewrite it without value level era witnesses generators? This des not seem to work for me:

let genCert' :: IsShelleyBasedEra era => H.Gen (Certificate era)
    genCert' = genCertificate shelleyBasedEra
AnyShelleyBasedEra (sbe :: ShelleyBasedEra era) <- H.forAll H.enumBounded
cert1 <- H.forAll $  genCert' @era

Yeah I think I agree with this now.