Ultimately, XML is a tool. It keeps your design work organized into practical compartments.
The easy nature of the language doesn't require massive amounts of knowledge or an alphabet behind your name. XML saves time and keeps the design flow organized.
I think so yeah, XML is still common.
But there is really not much to learn because you already know HTML and XML is very very similar.
Should you ever need more advanced things like XSDs or XSLT you can just learn it as you need it.
I believe Json is preferred over xml for web development. However in a general sense, XML is still used for a variety of purposes. For example, ebook/audiobook metadata is imported/exported in ONIX which is an xml document standard for the publication industry.