Zettlekasten is not new, but it's having a moment, at least among my friends. Someone named Sascha is writing a book on it and offering consulting, and his work has increased the popularity of the system. In short, Zettlekasten is a personal knowledge management system using notecards, created in the late 20th century by the researcher Niklas Luhmann. He wrote notes capturing knowledge onto index cards, then created links between the cards using a numbering system of his devising. Sounds pretty simple, right? It's actually pretty close to the research method I was taught in grade school around the same time that Luhmann was using his system for his work.
Like other productivity methods like the bullet journal, the concept is simple but prescriptive details are added to make it 'a system." The more details, the more you feel like this is a silver bullet. That's why I always find myself drawn to exercise routines that tell me exactly how many reps and sets to do, no matter what my personal situation might be. Like other productivity systems, the prescriptivism supports pollyannaish thinking and is shaded by hustle culture. Nothing is wrong with this system, but when you read the forums on Sascha's site, you see people hitting against the limitations of the system and blaming themselves. A lot of these people's problems would be solved by putting physical pen to physical paper to do a little thinking outside of the system, which, to be fair, Sascha himself does advocate.
Zettlekasten is a specialized use of hypertext, and in the 2000s, we collectively exchanged hypertext for what Mike Caulfield calls "the stream." For worse, in his opinion, and to the benefit of social media conglomerates and capitalists. Whether that is true or not, we did so willingly because of some very real frustrations with hypertext. Those same frustrations apply to zettlekasten and digital gardening. It is hard to communicate and learn through discussion with hypertext (at least, as we have used it). It doesn't reflect change over time. It is often very hard to navigate. Many of the things it does well can be replaced by the better searching algorithms created since hypertext's heyday. If you choose to use zettlekasten, it's worth thinking about how to best mitigate these issues, rather than sweeping them under the rug to sell how-to-zettlekasten books.