Thank you all for your answers.
We see that there are many distributions (even more than I thought). My idea was to identify if we should focus on 1 or 2 distributions in order to provide them with a good support and with a good user experience (installer, graphical desktop ...). With all these distributions, efforts are spread and from my experience, using Linux on these machines is not straightforward:
- Ubuntu Remix was nice but old and on package updates, I was unable to restart
- Debian installed on my PowerMac but failed on my MacMini and prior to than there was a long period with the switch to grub but it was not working
- I installed VoidLinux on MacMini but it was a bit hardcore to add the configuration to choose a graphical environment and then some of them had problems (I need to install the latest version with integrated desktop)
- I looked at Fienix and MintPPC but didn't really understand how to perform installation
@Borley I am convinced that big endian PowerPC (either 32 or 63 bit systems) is not the future and in parallel, I am studying how to acquire a Raptor workstation (I would like to be rather sure I will be able to have a minimum of time to use it) ... that would be a perfect powerful machine for my freelance activity (software development) and I also would like to test software, contribute to some projects, etc.
To come back to the topic, Linux on old PPC machines is for me at a hobby level or for testing.
For others, this is the opportunity to have a mainstream OS on an machines that already runs AmigaOS4 (X1000, X5000) or MorphOS (PowerMac, X5000).
People appreciate a distribution that is quite easy to install, have a good documentation (including about limitations), can be updated ... They don't want to go here and there to find tricks for installation, and then others for graphical desktops, and then again others for the graphic hardware acceleration ...
I will not consider distributions that are too much for experts (Gentoo, Arch ...) or that are not very active. One day I will have to learn more about BSD systems and test one but really I can't do that now.
@MPC7500 Unfortunately Void Linux will drop even 64-bit flavour in big-endian. Checking this point on Talospace, I noticed that ClassicHasClass wrote "The new BE Void PPC maintainer would be responsible for doing the builds as well as fixing issues, but it should be possible to coordinate hosting the packages on an official mirror. I imagine it's negotiable to do only glibc or only 64-bit or some such depending on the hardware or interest you have."