The reason for this message is that we had some problems making the packages on PostgreSQL 7.4. If you're able to use eZ publish without any problems on 7.4, continue to do so, but we're not prioritizing full support for all features using 7.4
eZ publish 7.4 needs more testing before we can support it. We had some problems during the initalization of the database. It will be supported in 3.4 definetly.
It should actually just be the installer ( when we init the database ) which is not supported with 7.4. You can initalize the database manually, then it should work fine. If not report your problems and we will fix it.