That depends on what you mean by relevant? History as an academic discipline has underwent immense changes since halfway the nineteenth century, which is around the time they published their most famous theses. While Marx and Engels their theories certainly have flaws from our contemporary understanding of both history and sociology - such as a lack of human agency, a lack of qualitative analysis and too much focus on economic determinism -, their work was absolutely revolutionary from a historiographical or sociological perspective. You have to remember that this was a time when studies of the lower classes or power dynamics weren't exactly a dime in a dozen, let alone works that were so well-crafted. So just in these two aspects alone their works were already highly impactful and their importance can not be understated.

So the impact of their work on historiography is unquestionable. Meanwhile, historiography itself is essential to understanding history. There's a reason as to why students of history have to also study historiography, at least at most of the European universities I worked with or visited. While this is mostly a course we give towards the later stages of the education, that's only because it can be a bit too complex for students who are still new to the world of academia. The way my university would handle it is we'd give an introductory course in the early years of the history education and then proceed to give a more advanced course towards the later years. We tried giving the complex course earlier, but even a lot of the third year students would struggle with some of the more complex theories.

The way historiography works is that contemporary historians build upon the theories that came before them. So while theories put forward by philosophers almost two centuries ago have obviously been nuanced and "improved" upon since their inception, that doesn't mean that they became completely irrelevant. They are without a doubt dated, but many aspects of their works are still very relevant. They revolutionized various academic disciplines for a reason and there is something of value there for everyone who is interested in historiography, philosophy or sociology. It's not just the contemporary academic theories which are important for our understanding of history, but also the theories that got us to the current status quo. That's why students of history have to study theorists/philosophers such as Marx, Engels and to a lesser extent Gramsci. To use a common English proverb : it's not just the destination, but also the journey.