I’ve done a little research, because my Mac Mini PPC G4 (7447A CPU) is running Ubuntu Mate. There’s far less attention given to PPC than the Intel and AMD chips, which is both good and bad (good because attacks are less likely to be widespread but bad because there’s less reliable information about their vulnerability).
I feel pretty good about the G4/7447A but there is at least one claim of a memory security breech demonstration on a G5. Read this page along with the comments:
https://tenfourfox.blogspot.com/2018/01/more-about-spectre-and-powerpc-or-why.html
There’s also much less attention being focused in 32-bit Intel and AMD cpus. From what I’ve read in other GNU/Linux forums, patches for 32-bit will be much more difficult than 64-bit!