No doubt we will go through a range of solutions to this problem, many of which will involve what other people, normally the local Spanish or Portuguese governments, should do to fix it.
Although forum members are a very small percentage of the pilgrim population, we can still make a difference by doing some simple things ourselves.
- While I am unlikely to leave behind toilet paper after I urinate, I do carry a plastic bag into which my rubbish is put during the day. That is largely fruit peel and wrappers from muesli bars and sweets. Women who might need to use toilet paper or tissues can do the same, or they can use some of the other, toilet paper free, approaches to feminine hygiene such as @Elle Bieling has already suggested.
- Every time I visit a bar for a coffee and snack, it is my practice to visit the toilet there. If you aren't going to buy something, leave a donation. Where I am staying now, the charge for the use of a toilet in bus and train terminals varies a bit, but expect it to be between .50c and 1 Euro. I don't expect an public toilet to be free in Europe, but I am happy when it is and they are clean as well.
- I have for many years carried the end of a roll of toilet paper and a small trowel, in case I do find myself having to defecate on the trail. I have never needed to use it walking on the Iberian peninsula, but I put that down to there being sufficient bars and cafes where I can use a toilet. In other places I have walked, one isn't always so fortunate.
While my observations over the years is that this is not uniquely a problem on the CF, and this thread was started by someone on the CP, other routes I have walked seem to have far less litter generally, and toilet paper in particular, than I remember being on the CF. It is a shame, but as forum members we can at least do something about it as individuals, and not rely on others to provide convenient solutions, much as that might sound attractive.