Richard Stallman wrote:
It is a good thing to boost awareness of free software. However, that expression has more than one possible meaning. It could mean that *all* the software running the server is free, or it could mean that only *some* of the software running the server is free. One cannot tell which.
We should encourage people to say things which are unambiguous about this.
I hope this means:
If a server used *only* free software, then they could say, "Powered by Free Software". Otherwise, they cannot in a general way covering all the free software they use, say "Powered by Free Software".
The others who use only *some* free software may never use "Powered by Free Software", because it would be ambigiously, and such users may have to say which of its software are free, and which are not free, using different expressions, to give the correct meaning.