You should never say anything which upsets people

You dont want to be upset, so dont upset others.

People can be upset by things which are both true and necessary to say. For example "you have X type of cancer and need Y treatment or you will die".

Also, there is no reason why people's free speech should be limited by what other people feel about it. People could technically be upset by ANYTHING depending who that person is, and if the government/society can police people's speech it opens the door for the weaponization of such policies and cultural norms to suppress things that are true but don't benefit the establishment.

The only form of speech that should be illegal is when institutions and their representatives lie to the public.

I disagree.

If I had cancer, I would prefer if no one told me so that I am not upset about it.

Self preservation is the most basic instinct of the vast majority or life forms, even if you would rather die than be upset that isn't the case for most people and beings in general.

It is also just plain idiotic to be even more upset in the long run when you find out you have cancer by suffering and dying instead of letting your doctor tell you and treat it before it gets that bad.

I disagree with all of your arguments. They are just lies.

I think if someone has cancer, he would rather not know, because no one likes being upset.

If someone has cancer, its better to just tell them it is nothing serious and they will be better soon.

You can still give them cure, but you should never tell them that they have cancer.

Or you can say that they have harmless type of cancer, so they are not upset.

Chances are that people will know if they're being treated for cancer anyway.

If you extend this logic to other situations you will see how ridiculous it is to say "it's not that bad" when it is life threatening. If someone's family member was about to die and there was nothing that could save them would you say "it's not that bad, they'll be back in no time" just for the person to find out anyway when their family member is never around again?

Telling people how bad the situation is allows them to be more prepared for it. If someone has severe cancer and you tell them it's minor, they aren't going to be as mentally prepared when they end up fighting for their life and undergoing extensive treatment and they will found out either way because of how extensive the treatment is and how bad having the cancer sucks.