Hello Zian.
To respond to your question, even though this explanation is late, EPSG publishes their database up to several times a year, depending on how many change requests they receive during a given time period. When they publish a new version of the database, they may deprecate (drop) old information that has been replaced with more accurate geographic transformations for example.
In the case of NAD_1983_To_WGS_1984_5 (WKID: 1515, accuracy 1.000 meter) that transformation was deprecated by EPSG so it no longer appears in the database or in their registry.
Esri, though, will continue in most cases to include the transformation in our software. That way customers who used that transformation in the past can reverse the transformation, then apply a more accurate transformation for the same area that might have become available. For example, a new transformation is now recommended by Esri for the United States and that is WGS_1984_(ITRF00)_To_NAD_1983 (WKID:108190, accuracy 0.100 meter)
That brings up another point. EPSG WKIDs are 4 or 5 digit numbers for all types of projection objects. In order to avoid any duplication or conflicts, when Esri receives a request to add new information to the Projection Engine, from a national government agency, that does not exist in EPSG, we assign a 6 digit number to the object - in this case the Geographic Transformation was requested by the National Geodetic Survey of the United States, and Esri assigned the 6-digit code 108190.
If you will access the article Dan Patterson mentioned above at https://support.esri.com/en/technical-article/000004829
you will see links to the supported geographic transformations for various versions of ArcGIS Desktop. These lists are important, so that you can check available transformations, and find the most accurate transformation that is available for your area of interest.
Note that the lists include the description name for the area of use, and also a bounding box in decimal degree coordinates, that are in GCS_WGS_1984. If your data falls even a little outside those coordinates the transformation cannot be used because the software will not apply it. The article at the link below describes how the software compiles the list of transformations presented in the Project Tool, or in the ArcMap Data Frame Properties > Coordinate System tab > Transformations button.
https://support.esri.com/en/technical-article/000012841
