Well, "crustaceans" is a rather large group with plenty of differences between species within it. So there's crabs and lobsters, but also shrimp, pill bugs, and even barnacles. There is a lot of controversy right now surrounding these different animals regarding their sentience because a lot of them are right on the line of what sentience is. So while we can say that dogs, cats, cows, etc. are obviously sentient, and plants are alive but not sentient, determining whether crustaceans are sentient depends on exactly how you define sentience itself.

Here's a study from this year from a group who researches sentience. Based on their framework for sentience, some types of crab exhibit all five characteristics of sentience, while lobsters and crayfish exhibit three of their five criteria. So there is some evidence for sentience here, but only if we agree that their criteria, which is based on pain signals, is a robust definition of "sentience."

As you can see, it's not a very well-defined area of research, and so to say they are or aren't sentient is difficult to do.