What particular AI software?
AI is a different name for statistics-related application processing, so whatever restrictions that might exist here are going to specific to the particular statistical software in use. Try it?
There were folks doing AI work on servers and software and hardware far older than this, too. I first ran into back-propagation and related programming in the 1980s, and it wasn't new then.
Xserve G5 PPC servers are very old and comparatively very slow, so there's almost certainly going to be issues with down-revision or missing software and probably also with performance, depending on exactly what you're processing here. Xserve G5 tops out at 10.5.8, and Xserve late 2006 at 10.7.5.
Frameworks and tools such as Core ML don't exist this far back, though.