One name I still remember strongly was "Ratimove". And as far as I remember, he used to add useful topics to his thread (I think his thread name was 'Encyclopaedia CMIIW'!) so that newbies could get useful information in a quick way, but sadly we lost that gem, and he passed away.
Ratimov had his nickname changed to
Symmetrick, went kinda nuts, started name calling, deleted every post he could, backed out of a sponsorship, and eventually
was banned. He single-handedly destroyed his reputation and left.
No one is indispensable. Although some people may be good and helpful to people in the forum but I never see issues in it if they were ban for later not keeping the forum rules. No one should be above the rules no matter how important you are. Well, although we may be losing some knowledgeable people but is only God type that doesn't exist. More knowledgeable people will keep coming to the forum also as some knowledgeable people are moving out. In here, people go, people come, forum remains.
I did not interact with Leo much and I had to take note of him when he wrote about his situation and exit from the forum, that was when I had to go through his posts, sincerely he was a great asset to this forum and was loved by many.
Some may live due to lack of interest or just too busy at the moment to be available on the forum. And this is something we can't do anything about. Everybody with his/her opinion about life.