It’s fallen out of popularity over the years, but reading programming books. The big ones. There is an expectation that a book will contain every bit of info about a technology, and you can learn it, in depth, in one place. Online articles, videos, etc., often just skim the surface. You don’t get that deep learning and facts that the books would have. I find even “Official documentation” online is sparse and often doesn’t include examples to gain understanding.

Unfortunately, the pace of change, especially in cloud services, cause books to be out of date too quickly, so I don’t see it making a comeback.