They still exist as a possibility in the real world, but it's not really fashionable to admit that assortive mating exists.
The fun part is going back into history, or imaginary worlds, and trying to convey what is, and is not, a mesalliance. There are some writers who seem to think that unless it's a scullery maid who's not a runaway princess, the prince isn't marrying down.
On the other hand, while marrying a foreign princess was always approved, it was only occasionally that marrying a noblewoman would be regarded as a mesalliance. It happened. One French king, near the end of the Dark Ages, was praised for marrying below himself to avoid taking a wife who was also a cousin. And Queen Victoria not only stalwartly defended the propriety of her descendents marrying the offspring of morgantic marriages, she was willing to countenance one daughter's marrying a subject -- in spite of stern disapproval from her German cousins, who would not countenance it in their own families, and it was, after all, the first time a British sovereign's child had married a subject for many a year.
Even within the nobility, marrying too far across titles could be a mesalliance.
Hard to convey neatly.
Still useful for creating the obstacles that are so vital in romance.