IPB v1.3 is freeware and does not require a commercial license and it
can be legally used in Cpanel as well with no problem whatsoever
so the information posted by another user in this thread is wrong!
However ...
IPB v1.3 is incredibly old, no longer supported, and hard to find and that
may the real reason for Cpanel dropping support for it.
For those who want to know, here is the IPB usage history by version:
v1.x - Any of the 1.x versions are freeware and open for public use without a license
v2.0.0 to v2.0.3 - Free for public use without a commercial license provided that you
do not make any attempt to remove the copyright messages or any of the links
to Invision Power from the footers or anywhere else in the software
v2.0.4 and above - IPB is available only with a paid commercial license and it is illegal to
use any version equal to or above v2.0.4 without paying for a license.
NOTE: The commercial license agreement for IPB was revised in 2.1.1 so that licenses
be allocated to a specific web address and the software can only be legally used
on the domain for which it was licensed. This means that even if someone actually
paid for a license, it would be illegal for them to use it on multiple sites or otherwise
a site that is not the one the license was intended for.