Hi Matthew
Trust Site code RLY20 was operationally closed (Status became inactive) on 31/03/2005. This is before the archive cut off date for the Org Data Terminology FHIR R4 API (31/03/2017), and RLY20 is not linked/related to any other record that is included in the R4 API, so RLY20 is not present/returned.
Org Data Terminology R4 INT environment is updated daily with the full scope of org data so mirrors live.
I will have to leave the eRS team to comment about their INT environment, it’s scope and update frequency.