What do you think about people who are constantly online and suddenly never come online again e.g. Chats, Youtube, Twitch, Discord etc?

Do you think such people died or did they get sick of it from one day to the next ==?

Rather the second. It happens very quickly when something more interesting suddenly comes into life.

More like they don't feel like it anymore, but there can be many reasons

Sick, deceased, accident
Something completely different from contacts is more interesting on the Internet
You looked for something on the internet and found it like a partnership
You now have a job and run out of time

And much more, have a nice evening