HALion 3 was released on the market in 2001, version 4 appeared in 2011, 10 years later. I owned version 3 and if I remember, very few third party companies produced libraries for HALion unlike Kontakt whose first version dates from 2003, I believe. At the time Native Instruments flooded the market with these three samplers: Kontakt, Intakt and Battery and many libraries from East West, Scarbee and many others.
During these ten years, Steinberg had to watch the parade go by and this was probably due to the fact that AVID bought Wizoo, producer of the HALion sound library. Many excellent synthesizers designed by third parties have had to be abandoned by Steinberg over the years and this probably has something to do with the rights. Moreover, we no longer find the HALion 3 library from version 4.
Here is a link for those who still want to use it: