Personally, I mostly text (or WhatsApp) and email with them. Sometimes if it's someone I've really lost touch with who seems to be doing interesting stuff now, I'll send them a LinkedIn message including my email address and tell them to hit me up. Sometimes I'll grab a coffee with them, but most of my favorite coworkers don't live in my town, so it's harder to meet in person. But I really think email is sufficient to keep in touch. I'll send links to stuff on GitHub or blogs that I think they would find interesting (or that I think they'd enjoy giving me a hard time for still being interested in). I also know people who set up Slack / Discord channels for keeping in touch. I don't do that, but I could see where it's nice.

Edit: the other thing I do is pass along interesting looking recruiting emails to people who I know are more on the market than I am. I figure they'll do the same for me next time I'm ready to move on.