At some point, someone did some research that discovered that people who have migraine with aura are at an increased risk of stroke.
The combined oral contraceptive pill also increases risk of stroke, expecially in the first 6 months. This has been very well publicised in the press every time it happens to a young person (or appears to happen there is often no proof that coc are the cause, only circumstantially that they had recently started taking them). If you are young then your risk of stroke is very, very small indeed so that the very small increase in risk is more significant.
In 2007 the guidelines on contraceptives changed. They put both these risks together, and came out with the recommendation that women who had ever had a migraine with aura in their life should not take the COC pill. This changed from prescribing with care (ie the prescribing doctor weighed up the risks and made a decision).
I took COC for about 29 years (without taking out the time I was pregnant or bf) and was suddenly taken off it in 2012 at the age of 49 when my GP surgery suddenly caught up with this. Of course, I would have had a stroke if I had continued for the remaining 6 months before I was 50! I even had to throw away the ones they had just prescribed.
This, however, has nothing to do with hrt. HRT is replacement. Your risks of stroke from other causes are way, way higher. The triggers for migraines are varied and some people are better with hormones and some are worse. Getting the dose as even as possible without ups and downs is the best thing you can do, then you can decide for yourself if it is better for you, or not.