holiday insurance, after being told all of the cancer is gone, and no treatment needed

“there are some mainstream travel insurance companies that only ask certain questions, like are you receiving treatment not about what you have had”

There may be but do read the t&c very carefully as most will have a section on pre existing conditions or recent treatment. In other words, just because they don’t ask you about your medical history doesnt mean it’s automatically covered. 

I agree, it's essential that everyone checks exactly what the policy wording says.

There are some I've come across that appear too good to be true and usually they are, as you say excluding significant pre existing conditions altogether.