The postgres community has stopped all support for pg 8.4 in July 2014. Support for pg 9.0 stopped in Sept 2015. It is certainly not recommended to run systems on unmaintained database systems with security holes. We do not have the resources to test openacs on old versions of pg, and to program around bugs that were fixed many years ago.
The HEAD version of OpenACS is essentially OpenACS 5.8.*. The fact that it does not complain about pg 8.4 says nothing that it works with it. The head version is not a released version (and never was). Currently, all development happens in the oacs-5-9 branch. When active development stops in oacs-5-9, the version will be merged with HEAD. The version numbering of HEAD acs current release + 1 is needed for the apm management.
If some of your installations work with pg 8.4, then remove the check on your installations, but you have to do this on your own risk .... unless there is a volunteer who provides support for pg 8.4.
