What should I be learning?

A couple of weeks back, I was approached by a startup that interested me, and it was the first time in a while that I applied for a technical-only role. In the interview process, I failed during the technical because I follow Object-Oriented Programming whereas they are keen on only hiring Functional Programmers. Of course, this was a blow to my ego, but it was also a refreshing reminder that I haven't been up-to-date with engineering principles. Back in my day, Functional Programming was a mostly academic, but nowadays it seems to be actually used by companies!

In the last couple of years, I have been working in a large org as a Senior Software Engineer. This has been a real change to my previous work experience, where I worked exclusively in R&D. Working in R&D forced me to work with newer technologies, and more importantly, it forced me to be continuously learning about what is going on within the tech hive mind. My experience with Apache Spark from the time it was incubated is one of the reasons I have the job I currently have. But in a large org, things move much slower, and I have found myself outdated/an absolute dinosaur.

What are some things that you believe are important to start learning now? I am looking to stay mostly technical for the next 5-ish years, but I want to update myself with what is perceived to be good engineering principles.