This idea might apply to other BMC products but I am submitting it with Remedy and ITSM in mind. I went to look for an updated Entity Relationship Diagram (ERD) for ITSM 8.1. I came across this page https://docs.bmc.com/docs/display/itsm81/Foundation with a discussion stating one is not available for 8.1.
An ERD is available for ITSM 7.0 http://documents.bmc.com/supportu/documents/57/43/65743/65743.pdf. It seems that if an ERD is ever released that all later version documentation should also include an ERD.