Hello Melita,
Thank you very much for your clear answer.
After I posted the question I tried to compare contents of EPSG standard dataset and ESRI projection engine. It was quite easy to compare CRS as arcpy has a SpatialReference() method that can be used, with an EPSG id as argument, to test if a specific CRS is supported by the projection engine.
This way I noted that many CRS are missing also when testing ArcGIS 10.5.1 against old releases of EPSG dataset, like 7.9 that was released 10 years ago.
If I clearly understood your message it seems that when you include information from EPSG you apply a filter to extract only those considered relevant.
Testing the Datum shift operation is a little bit more complicated, but from your answer I assume that also in this case any projection engine release supports a subset of the operation available in a specific EPSG dataset release.
Then, getting back to my original question, the goal of having a one to one match for ArcGIS projection engine and the legacy application using EPSG dataset is impossible.
I think I can try to extract a set of information about CRS and Datum shift operation used in the legacy application and not supported by an ArcGIS release and add them as custom CRS or operations.
Thank you very much again for your detailed information.
All the best,
Stefano