Responses
While I agree dirty water is the more important issue, I just wanted to throw in that there are plants that have been used as soap substitutes for ages. Its not just a fantasy trope. Soapwart leaves steeped in water make a very nice liquid soap and was used often in medieval laundries. People didn't bathe often back then but clean clothes were of utmost importance even to the poorest peasant.
If you read through every arc, you'll notice that the climax of that arc is resolved either by something never mentioned prior, or by some trivial means that hand waves the complexities of the issue.
I mean, I got the feeling when the manga suddenly brought up the civil war issue despite not mentioning it at all before Claude. I mean, MC even said her main motivation was to avoid the busy life of an empress - that after remembering her previous life's memories it was her ambition to 'live the ultimate leisurely lifestyle'.
Then there is the life eater incident. MC can somehow detect the flow of something in blood perfectly, and isolate what is very likely a set of microscopic organism (remember, it was small and light enough to be airborne) despite never having used her powers in such a way before (She's surprised by how she can feel the blood flowing). This is like saying a genius programmer can code in binary - maybe they can, but you really can't expect IBM Watson to pop out of that. She even says that she isn't that gifted magically. The author clearly hasn't done any sort of research but I guess at least they aren't actually using any real life diseases.
Also, I find the hand washing policy sort of hilarious. The author uses fantasy nuts - easily available and cheap to boot!- to avoid addressing the issues with actually making enough soap in a time where mass production isn't a thing, but completely ignores the issue of clean water - something a lot of places struggle with.