I have had online friends who were extremely dear to me disappear without a trace. The closest parallel I can draw on was some years ago shortly after the outbreak of the Syrian civil war. A dear friend of mine was living there and I'd been providing him with technical assistance to get online anonymously and securely and seek refuge in a neighbouring country. He made it chillingly clear that if he were caught going online, he'd be shot on sight.
He regularly logged in when daylight broke in his local timezone, until one day, he didn't. It wasn't unusual for him to go a day, a week without being online, especially as it became harder and harder to access the internet from the middle of a warzone. Weeks passed, then months, then a year and eventually two. I tried to dwell on it less and less as time went by, but there wasn't a month where something didn't remind me of my dear friend, had he made it to safety, had he been killed, had he killed himself as he'd sometimes mentioned, to avoid the risk of being tortured? The lack of closure was a painful barrier to grieving.
Eventually, I'd had enough and decided to track him down. It took me a while, but by that point, I was working as a private investigator and so I had resources and some experience investigating missing persons. I found an account linked to him, he'd changed his identity and was now living safely in a different country. I was overcome with a terrifying mix of relief and anger.
Without thinking, I blasted off a message damning him for not getting back in touch with me and how could he put me through all of this shit! He apologised and explained the difficulties of his escape and how he'd been unable to contact me - my old online identities abandoned long ago. But he need not have apologised, it was horribly selfish of me to demand an explanation - I felt entitled to one, even though I wasn't. He thanked me for what I'd done for him, I've kept that message in the years since and I deeply treasure it.
So speaking from personal experience, I completely support what @GoodPersonEffed has said. An explanation, a caring goodbye and perhaps a little reminder of what they mean to you and how they've helped you, keeping it positive and providing that closure that people naturally crave.