Lacking (external) identifier in ClinicalImpression

I intend to use ClinicalImpression to represent some data from an external system.
I noticed that ClinicalImpression is about the only resource that does not have an identifier element to represent external identifiers.
Is there a specific reason for this omission? I tried to find one in the docs, but couldn’t. Or is it simply something that is attributable to Maturity 0?

ClinicalImpression does have an identifier in the soon-to-be published STU3 (here is a preview using the current build: ClinicalImpression is still FMM=0, which means it is effectively a draft and subject to change.

Thank you Michelle - of course I should have checked STU3, thank you for the pointer!