I have pre existing medical conditions that are declared to my insurer each time I take out a policy. These conditions (or situations arising directly from these) may not be covered but everything else should be.
Undeclared medical conditions may invalidate an insurance policy - I'm on a FB group for people with my condition and there was a post today about not declaring it - what's the point of getting insurance if by not declaring a condition your insurance might not be valid and they can reject any claim.
I sympathise with them but can't help thinking if you're going to the bother and expense of taking out travel insurance at least you should make sure it covers you in the event of disaster - even if it costs a few quid more to declare an existing condition.