You're right, if homeowners downsize they'll lose out with lower prices. People don't downsize very often.
But what policies are you talking about? How can the answer be anything other than increasing the supply of housing (or decreasing the demand i.e. the population)? Prices are only as high as they are because people pay them because they don't have any other options. Rent is high because demand is high relative to supply.
The only thing I can think of would be higher taxes specifically in places with high house prices in order to fund huge investment in poorer areas to make them more attractive to people and businesses.
Because it's too complicated. It would be too long of a list for people to remember and its too difficult to prove the harm of individual ingredients and they're probably almost all fine in moderation.
Look at where we are now with saturated fats: Every major health organisation in the world says they're linked to cardiovascular disease and should be limited in diets, and meanwhile hordes of people who've read a pop science book or watched a YouTube video think they know better and can eat all the fat they want.
We've tried going against fat, we've tried putting the sugar, fat and calories and packaging, people know about calories in, calories out, and yet obesity never stops growing.
UPF is about the manufacturing process. The idea is that it isn't going to include the things that you make in your kitchen from whole and processed ingredients, but it does include the cheap easy to overeat stuff cooked up by food manufacturers.
Also I'm not aware of anyone who says you should eat no UPF whatsoever. It just shouldn't be a huge part of your diet.