There isn't any postgresql version in the 9.x series newer than 9.4 listed in the matrix.
There is PostgreSQL 11 in the matrix. "9.4" stands actually for all PostgreSQL versions from 9.4 up to 11. The matrix does not enumerate all versions, just compatibility boundaries, where e.g. some support is dropped. I am running e.g. oacs-5-10 with pg 12.2, openacs.org runs still with pg 9.6 (but also oacs-5-10).
The recommended action is to upgrade to the 5.9 series (oacs-5-9), this will provide long term stability. The upgrade from 5.8.* to 5.9 is not hard.
Compatibility in the matrix means the full spectrum, from installation, over upgrades to runtime support. If one has a running version, and is willing to rewrite update scripts, and do not care about security updates, responsive design, etc., then the old version can be used probably a long time. But if you want to upgrade then, it might be harder.
