I would have to assume there's some ecological pressure they're either experiencing or shielded from. If they're under environmental pressure, they are either exceptionally developed to endure environmental changes or they're adapting to the environment in ways that don't reflect in their physiology. If they've just found a niche biome where the ecological conditions are fairly static, and they're well suited to the environment, what changes would you see other than some generic genetic drift?

Yeah exactly. I’m not a biologist but I am aware that horseshoe crabs are harvested for their blood and its unique properties. How much evolution (in the last 450MY) have they undergone in terms of their immune system and other microbiological processes? We may never know but I think it’s quite a stretch to assume “none!”

I came to mention the same. Static phenotype ≠ static genotype.

It makes sense. I just wasn't sure how likely it would be for species to evolve in significant ways over a long time without obvious changes to the shape of their fossils. Difficult to spot evolution happens a lot, apparently:

Cryptic, or sibling, species are discrete species that are difficult, or sometimes impossible, to distinguish morphologically and thus have been incorrectly classified as a single taxon. Cryptic species are found from the poles to the Equator and in all major terrestrial and aquatic taxonomic groups [2, 3]. For example, a recent meta-analysis yielded 2,207 articles reporting cryptic species in all metazoan phyla and classes, including 996 new species in insects, 267 in mammals, 151 in fishes and 94 in birds [2].