It seems to me there are two types of protest songs that become classics: the timeless lyrics, such as Bella Ciao that was mentioned earlier; and the lyrics that are open to being endlessly rewritten, such as Rigs of the Time.

The oldest fully applicable protest song I sing without changing a word is from about 1850, but there are some very good contemporary writers about now too.

And I can't resist adding my favourite Matt McGinn line as he's just been mentioned:

They're never ever satisfied, though millions they have made,
But still they think that you and I are grossly overpaid.
    There's nothing that they widnae steal,
      they even grudge your kids a meal