Such systems are quite old; in German they are often called “Zettelkasten” (or, in plural, “Zettelkästen”). In 2013, there was even exhibition about this in Germany, and a book was published - ‘Zettelkästen: Maschinen der Phantasie’, which is hard to find now…

I use no notecards, but a few text-files, because that is way faster and easier to search through. So, everything I write down by hand, is later filtered and, if it seems worth it, put in such files and saved.